I'd like my Sims to be able to swim/dance/exercise/overeat/etc. without their weight and muscle being affected. I suppose this would require retuning each action individually, but I have no idea where to start. Does anyone know how I could achieve this? Thanks in advance.

Some suggested existing mods to look at for a start, all at the Sims Asylum (regsitration and login required to actually see anything there). Note that you can't use these all together as they will conflict with each other, but if you look at the discussions and examine the tuning changes in S3PE, you can get a sense of how complicated or simple you might want to make this.

Shimrod's No Muscle Gain
TwoFtMama's Lose Weight Gain Fitness Slower
Bluegenjutsu's No Athletic Skill While Swimming (maybe that one is not as relevant)