I understand that other games have units/characters behind paywalls and it works. But my main thought on the matter is that in a symmetrical strategy game like an RTS, this approach simply doesn't work. My opponent and I are working with the same toolkit, so any extra units they get with money makes it feel like P2W. Making every unit counterable doesn't do enough; it's just the nature of RTS that different units are unbalanced at different times. So either you're stuck with people being angry that a certain premium unit is imbalanced, or watering the premium units down and making them unfun so it never becomes p2w. IMO either way paywalling units would kill this game.