But Paradox is wrong here. It should be impossible to intervene in a convoy battle even with escorts. Convoy raiding battles were very short. The famous Duisburg Convoy battle lasted only 40 minutes! Enough to trash 1 DD and 7 Italian convoy ships.
The timing of naval battles in the game is somewhat abstracted, to enable players to have some kind of participation (even a "long" surface battle would generally be less than one day, which is a very short number of turns in HoI4, and easily over before a player will have noticed it has started. I understand their move here to give players agency, and without all sorts of pauses, notifications, and 'games-within-a-game' I can't think of a system to give player the kind of agency they have now when it comes to naval battles. So it's not "wrong" per se, but more (I believe - I could be wrong) a conscious design decision to accept that the way it's simulated isn't entirely historically plausible, but more fun for the player (and it's not the only thing in the game like this, by any stretch - that all your generals and units do what they're told immediately in most situations, or that troops insta-unload from ships in one hour, or all sorts of other things, are understandable concessions to gameplay at the expense of realism when it comes to mechanics).
It gets even harder to simulate with battles going through multiple day-and-night cycles, as night actions during the period were, generally, very different from daytime ones!
Whats your balance for low level (1->3) ports vs major naval bases for this? I used to have my strike forces near the combat zone, but noticed I was wasting months because my fleets were retreating to level 1 ports vs. repairing at major bases.
I suppose the ideal would be "base near the front line, but repair at major ports" but I struggle to get the AI to follow this. As a result I just block most minor level 1s.
I usually block off the reports for repair that are too small to be useful. You may well already know this, but I'm 90% sure that if you block a port after it's started repairing, the ship will reallocate to a port it can repair from, so if you notice a stack of ten sitting in a dinky little level 1 port, turn it off and they should scoot to the next nearest port. It's very important to turn off ports for repairing that are under serious threat of enemy air strike.
Also regarding patrol fleets, what is your normal build?
I only play single player, so I don't have the tough-as-nails competition of MP to hone my builds, so I wouldn't necessarily pay a heap of attention to my general approach, but if the ships are available, then I'll usually go with 1 CA, 2 CLs and 7 DDs in my patrol fleets, and then the strike force is 1-4 CVs, and then the whatever capitals are available, making sure there are enough screens (I usually have 4 times the number of capitals as screens, so that the fleet doesn't need micro-ing after each loss).
Also, does convoy escort grant spotting? I have a number of 4-12 sized old destroyer fleets on convoy escort and wondering if they would pick up the enemy.
I don't know - good question! I don't think convoy escort fleets engage other fleets (I have no memory of this happening), but I haven't tested it. I
expect it doesn't happen, as in my recent UK game I had a lot of escort fleets and I'd imagine there would have been more battles if they were spotting, but that's a guess.