I hadn't actually thought about a feedee upgrade system! It's an interesting idea.
I might add the ability to create feedism porn to generate income whilst you're immobile.
It would make the game feel more robust but it's only if it's in your interest. At the current moment, if you interact with a feeder there's no way to really level up your OWN feeder perks aside from self control obviously, but I'm also of the impression that if you continue to deny them dates they'll lose interest. For me at least, this means that I kinda wanna hold off on even MEETING feeders until I finish my OWN feeder perks and then indulge in gluttony after I've made enough passive income from Dizzy's.
Oh and speaking of I do think there's some bugs for being a feedee and getting meals from feeders. I'm assuming the feeders are supposed to pay always but certain events like the town food festival and the bake sale still deduct money from your total.
That does make sense, it'd help make being a feedee feel less tacked on if you could earn XP and upgrades from gaining weight yourself. I'd like players to be able to play how they want without feeling like they're missing out by doing so.
Alternative ways of making money could help, so you don't need to rely on the doughnut shop or feeder upgrades that boost money earned.
You're right that it didn't make sense to still have to pay for those events when with a feeder. One of the bug-fix updates I released this weekend addressed that, so hopefully those are showing up as free for you now (when you're there with a feeder).