If base weight will be highest weight reached there is no chance for naturally skinny characters that have to be encouraged to gain weight. That would be a pity because encouragement is such a nice fantasy!
imho base weight would have to be more sophisticated, more like "metabolism" that is very hard to change, if at all.
PS.: even if your body does not like it, you will lose weight if you do not eat, but also lose muscle ie.: fitness. No yo-yo without calorie input.
Yo-yo could be implemented as a "next meal buff" after starving ...
BTW.: What you call "minor improvements" feels like a major milestone!
regards,