I’m not asking to join (I’m busy with my own groups right now), but I am curious what your group had in mind in terms of a slower weight-gaining. With most adventures taking place over relatively short periods of time, meaningful natural weight gain is difficult to pull off.
I also want to point out my own obesity rules I’ve (slowly) been working on. They’re for Pathfinder, not 5e (encumbrance penalties would be the main thing that’d need to be adjusted), but since I wrote those with a similar mentality as what your group was looking for (less fetish and more just optional rules) it might give you and your GM some ideas.