This probably isn't worth mentioning but I will anyway. I didn't recruit a bazooka or a tank into my army because I upgraded regular troops for the few times I used bazooka people and I use the tank destroyer instead of the tank.
I would have had to play the first few missions again to get to those points had I not had a back up save for the last missions in Part 4.
Those 2 are technically missable I guess.
You can go back as far as you want, but you're starting over from there. Anything else is getting re done
I used the tank destroyer too when ever possible
And artillery at every opportunity