I never used any tanks for any fodder until they added the bonus thing, with which I decided to casually experiment. I would simply scrap them all for the currency, which I really needed at that time, and in fact still do need. When I did start to use the bonus XP, I wasn't one of the ones to farm up thousands of Renaults...I still haven't gotten enough of them to hit the original 20% on my first one. But honestly, if the devs were going to implement a bonus XP thing AND a node where you always get the same tank for completing it, what did they expect people were going to do?

And yeah, I prefer Rena to Tabby by quite a lot...except the unlocked skins don't actually look like the same woman as her 'tank' form.

As for the oil costs on PoC, I don't know how I'd approach it. Maybe remove oil costs for initial attempts, but limit the number of oil-free attempts to complete each level to three, and if you want to hammer against the RNG after that to clear the level it'd start costing you oil.