doesn't give reduced leader experience (divisions must be at least 8 battalions)
Are you saying 8 battalions is the minimum to 'not give reduced leader experience' or something else?
there is merit to sending two divisions of "tanks" to Spain (2 tank and 6 cavalry) out of your seven volunteer divisions. These are the cheapest you can make a division that both counts as "tanks" and also doesn't give reduced leader experience (divisions must be at least 8 battalions). You do it for the purpose of (a) not grinding infantry leader, and (b) grinding/not grinding panzer leader. 3/7 meets the 40% required to grind panzer leader, and 2/7 won't grind any "X leader" traits. Switch around your "tank" divisions to something else when you're about to get Panzer Leader so it stays at 99% until you're ready to finish off all the traits you're looking for, then switch them back to tanks for that last 1%.
Party this is a response to you but partly further info if anyone wants the tips/detail.
I agree there's merit to sending tanks to Spain for exactly the reasons you note, but I never do it 'first'. I wasn't as clear on this as I could have been above, I left a lot out to limit the size of my already-long post.
Personally, I usually switch one or two infantry divisions to tank divisions (in the field) in early '37 once I have the XP to do it.
There's several reasons I wait ... to save fuel ... to get more XP from the same battles/time (your early tank divisions are tiny) ... to wait until I have the XP to create larger tank divisions ... to manipulate the division so it has less equipment to free up PzIs for conversion to SPG2s in '38/'39 ... you can still easily max-out the unit experience starting your tanks later in Spain.
As I noted above, from what I've seen with lots of play testing (using detailed spreadsheets to track experience gain of all forms - unit, army XP and leader traits) manpower is the primary factor that determines how quickly you gain Army XP (equipment does seem to play some role but I've never needed/bothered to test exactly how much, it's just a lot less it seems)... number of divisions in combat (not manpower or size of divisions) impacts how quickly your leaders gain XP (along with other things like how many traits they have and if the Spanish are 'helping' in the battle -both of which drastically reduce trait gain speed) and the speed at which your units gain unit experience remains the same regardless of the above factors, all that matters is whether they're 'in combat' or not.
So, instead of sending tanks OR infantry early, I personally send 7x 18w motorised divisions first up. Again, I didn't go into this above to limit the size of my post but anyone can work this out if they realise 'manpower engaged in combat' is the most important factor in Army XP gain. With my early mot-divs I give only one of those 7 divisions trucks. The only reason I have one with trucks is to have one fast unit to 'rush' this provence:
The Spanish usually haven't manned that provence early, and it's awesome for farming leader XP, especially early because you can put all 7 divisions into this combat meaning I'm getting: cav leader, engineer, trickster (3-way battle) and hillfighter simultaneously (for both my leader and field marshal. If it wasn't for this I wouldn't send any trucks with any mot division to save fuel.
I strategically redeploy everywhere in Spain (and a lot in China too) because it's faster (much much faster with the malaise) and to avoid any attrition when crossing mountains.
The reason send motorised divisions without trucks is... the 18w mot division has the greatest manpower of any starting division template Germany gets. By not giving them any trucks, I don't use fuel and don't need to build the trucks ... but I still get XP from these units (including cav leader XP). While this leaves your units drastically under-strength, this is actually a GOOD thing when you want to prolong battles and the Spanish forces are so weak, combined the the malaise penalties, you simply don't take losses, but you DO gain a lot more early XP. You can see in my screenshot, even with drastically under-strength units, I've still got as many as I can attacking across the river to further reduce their strength and further prolong this combat. Once the defenders org drops to the point where he's close to retreat, I'll stop for a while and attack elsewhere while he regains org, rinse and repeat. The hardest thing is to stop the Spanish 'helping' and getting in the way of your wonderful XP farm.
Using 7x of these early mot div you gain ~0.7 Army XP per day instead of ~0.5 per day with 7x your 18w infantry ... or less if you send those early tank divs.
With all that Motorised, I train Germany's one level4 Cavalry leader general. I save any XP until he has the Cav trait, I train him first because after he's got Cav trait I switch all my motorised divisions to infantry and do not field any more motorised in Spain or China.
By the time he has Cav trait, I've gained around 20-25 army XP, I let him run a bit longer until that's 30-35 XP. I then use that to switch all the motorised divisions to infantry divisions ensuring the template uses (at least slightly)
more manpower. I never go backwards on the manpower I'm fielding in my Spanish divisions because the only time I'm not fighting in Spain is when I'm waiting for manpower to arrive (and manpower takes a relatively long time to arrive). In that downtime you're obviously not gaining any experience. After this first change I usually have 7x 22 width infantry divisions each with 4 support companies. I then train infantry leaders and have a lot of stop-start battles where I fight until I have 5 or 10xp and spend that increasing the size of the infantry divisions.
I stop once I have 7x 42 width, 5 support company, infantry divisions. I get there by late '36. Again, I do not fully equip these units. I often leave them with only their starting equipment (what they had as 18-width divisions). The lack of equipment has no negative impact on losses or XP gain in Spain and again the lower combat strength is helpful to prolong battles.
Once I have my 7x42 width infantry all in combat, I'm now maxing out my daily Army XP gain at 3 per day. I'm still training only infantry leaders at this point, switching leaders as desired.
Once I get to the point that I want to start training Panzer Leaders or manipulating leader trait gain in the ways you describe, I then a 40+ width tank division template, I carefully design this to ensure manpower stays roughly the same as the 42-width infantry division. This is normally done in early '37. There are several different ways to create the tank template, to manipulate it so you don't have 100% equipment. You can literally have your tank division with zero tanks or trucks and still gain the same unit, army and leader XP. I don't get that cheesy, but I don't ever take more than 50% of the tanks, and always take only PzIs to Spain (saving my PzIIs for China).
The other thing to know is, so long as you have 25% of your battalions as tanks in the tank template, it's considered a tank division (and you get Pz Leader experience gain from the unit). You can run two of these plus two infantry divisions under one panzer general for faster pz-leader trait gain. Even if you fight with only those two infantry divisions, the fact that leader also has two Pz divs, means he gains pz leader (not infantry leader) when those infantry fight.
There's still many points I've left out here otherwise my posts would be longer still. Such as how to manipulate your tank divisions so they receive less than the full compliment of tanks when changing from Inf to Tank divisions in the field, or how to maximise your leader trait gain by stopping them training a particular trait just before they get the trait, or how to manipulate where the malaise will be applied in Spain (for example, if I can, I want to delay the malaise being removed from the provence pictured above for as long as possible, and there are ways to influence this).
Min-maxing allows me to end up with 19x max-experience divisions ready by '39.
I normally go for:
6x Paratrooper 20width
4x Tank 40w
9x Infantry 40w
and lots of planes
I also have all my main generals and field marshals with at least one (in most cases two) terrain traits, trickster and infantry/panzer leader.
And most are at least level 5, some are level 4 or 6.
I also have heavily upgraded: PzIIs (I use light tank recon sometimes, esp with Para divisions and about 3-4 PzII divs throughout the early war), PzIIIs, Light SP Arty II, and planes: Bf 109, Nav bomber (range maxed) and CAS (range 3 upgrades).
With the upgrades my Light SPAII actually have more soft attack than vanilla Medium SPAI, and they're very cheap and quick to produce by converting PzIs and similar captured tanks. The lower armour is more than compensated for by my upgraded PzIIIs.
I also have enough XP to always get the bonus when doing doctrine research.