I agree! I think something like this is becoming necessary now that the game lets you simulate being both a feedee and a feeder. Especially as upgrades that boost your working income and help with relationships are locked behind feeding girls.
What I may do is add XP gain to lots of different actions (including gaining weight yourself & exercising), then perhaps have four separate skill trees: Feeder, Feedee, Relationships & Fitness. The latter of those would help make exercise, weight loss & muscle development easier.
I'll probably remove all upgrades that boost income entirely and replace them with a better employment system that has multiple jobs, promotions, etc.