Oh, you are right. My bad. Still, triple amount of original XP is much better cosidering grind times.
Here is screenshot, you can see formula:
/original/3X/9/6/967f97d0b948be91a6c9b8cc79005430f9197180.png
Also still have to level those other squads and soldiers.
Unless you buy extra slots you dont really need to. Squads you get at the end of campaign are the same as the ones at the start, but better. So its kinda pointless to grind early squads if you dont have slots.
Realistically speaking, chances of you using only premium squads till the end of campaign are pretty slim, but even in that kind of situation, having zero lvl squads with max lvl campaign is not a problem at all. You will have enough bronze to equip any squad with best weapons and there wont be enough high level players to overpower you with bigger numbers.