Scaling to commander level wouldn't be nearly as bad if your tanks leveled faster than commander WITHOUT needing dupe-bonuses (this way, you wouldn't feel too discouraged at the idea of bringing that new 5* onto your team, because having her be dead-weight for about 10 battles wouldn't be so bad, but the current setup of like 40 battles most certainly is)

Oil cost of invasions should be halved, at least. They're too damn pricey, and at level 50 cost 386 oil per pop. 2,316 oil for all 6 is MORE oil than you get for 12 hours of automatic oil generation (2250 with the now 5th oil well), and even with scouting tossed in, it leaves very little room for Factory, PoC or random leveling