Been playing some USSR game with ver hard settings, trying with different set ups, on the current game I am focusing on INT, CAS and ARM, ARM mostly consisting on ARM/MOT/SPART/AC, but maybe its to weak, will prob change to ARM/MOT/MOT/SPART.
I am trying to stop Germany on the border with slow manual withdrawal.
Any tips and inputs are welcome even for future games (USSR) with different focus and setups.
DIsclaimer: I'm not an expert on playing the game on very hard, but have played a lot on normal difficulty, and a little on hard difficulty. Lately I've also been playing mostly as the USSR.
That said I can quite confidently state the following: Arm, Mot, Mot, SP Art (triangular) is better on the defensive as a unit than Arm, Mot, SP Art, AC. (binary) This is valid for all triangular vs binary comparisons of equivalent units. There is one caveat, however: Frontage is limited. If you have 10 frontage, you can get 5 binary Divisions or 4 triangular Divisions into the fight, a two-pronged battle with 15 frontage allows for 8 binary divisions or 5 triangular ones. Of course stacking penalties reduce this advantage a little, but then again your binary divisions do have a slightly higher combined arms bonus. This is only an issue if you consistently have 5 or more Divisions available to defend a province, with reserves ready to be sent in to face larger multi-axis attacks.
Now, you write that you're concentrating on Int, CAS, and Arm. On the aviation side this makes some sense as you get a lot of bang for your buck, especially against panzer divisions. You will be constrained by air base space before you know it, though, and the short-range CAS needing to be based very close to the front do add strain to the supply network. Most of the units you will face are relatively soft, and Tac is definitely the best at dealing with soft units (but the most dangerous ones are hard). At some point you'll probably find that the only way to increase air support, once you already have a lot of CAS, without ruining the supply situation is to build Tac and base them further from the front.
On the land warfare side, I think Arm is probably the best bang for your buck as far as tanks are concerned. However, motorising your entire army is going to be counterproductive as motorised and armoured units use up more supplies, and especially fuel, and they are more expensive to build, so you can field significantly less of them. Considering the hefty economic modifiers of the Very Hard difficulty, I do wonder whether it is even possible to IC-build your way out of this limitation, and I'm pretty sure it won't be possible to infra-build yourself out of the supply issues.
I would suggest building your army around infantry divisions, either triangular ones (more defence focused, and you need fewer units to cover the whole front), or binary ones (more offensive focused, packing more offensive punch but you need more of them to cover the front.) Then you can supplement this with a mix of offensive tank divisions (Arm, Mot/Mech, SPArt, AC/TD) to break through the enemy lines, and defensive tank/motorised divisions (Arm, Motx2, SP Art / Motx3, SP Art) as a fire brigade to stem enemy armoured offensives and to exploit breakthroughs (as in protecting the flanks of an armoured spearhead consisting of binary divisions).
In short, the Soviet Union's massive manpower reserve gives it an edge, use it. (at least to some degree, don't just spam Milx4, because you will loose and probably run out of manpower)
Finally, to add even more punch to your divisions, consider researching the superior firepower doctrine, which allows you to add a fifth brigade to your existing units.
Here is an overview of a Germany Player's OOB to beat the AI on Hard difficulty. This player went for binary divisions all-round, look at the ratio between tank forces and infantry and the total amount of units, and that's for Germany, which is short on manpower, compared to the Soviet Union:
https://hoi3.paradoxwikis.com/Barbarossa_OOB
In any case, I would be interested in seeing the results of your build on very hard difficulty.