The problem with Commander level is it pulls in the opposite direction of the tank replacement the game also does.
Which all boils down to limited single tank leveling options.

Sticking with the same true and tried front line, and thereby having a leveled front, solves most of it for invasions, since it matters a whole lot less what all other tanks in the force are if the front can take and give enough to simply make all else follow ups and not key players.

I run with 4* Tiger, 4* T34/85 (from day 1) and Max DPS focus fire tanks, Invasion causes me little trouble for that reason.

Oh sure, I have some prestige 5* around, but they are in the second row. The 4* Pull the heavy load, and I have cleared all missions and have last oil well. If I wanted to front those, I'd be in trouble, which is of course the opposite of what should happen, but am pragmatic enough in the short term to go 'oh well'.

Number of invasions and oil is a luxury thing. Could be fine tuned, maybe even should be, but honestly, it is an optimization not a broken part of the game. More pressing items to get fixed - Such as realizing that only running events at holidays is leaving waaaay too large gaps for the base game to cover, more so as the base game has already had to do cover for 3 out of 4 months.