Dark side corruption

i had some questions about the corruption that happens to one's body when they use the dark side.

the dark side twists one's body unnaturally, but does it make you weaker/sicker or is it merely just an appearance change? for instance, palpatine in TCW is able to fend off maul and savage oppress fairly easy, despite being a ragged old man. he's also still clever during ROTJ because he leads the rebels into a trap, which shows that he still has his mind in perfect health.

i feel like the dark side alters one's body to be hideous on the outside, just as much as they are on the inside. outside of physical appearance, the person is healthy. palpatine lived into his 80s, which is 'doing well' for most humans. however, he has looked decrepit since episode 3 (and probably before then).

i also noticed that corruption happens depending on one's actions. palpatine was an evil man, so he changed the most. count dooku on the other hand seems much more chill, and he wasn't changed at all despite using lightning on multiple occasions. vader had glowing eyes because he killed kids and stuff, but he wasn't as drastically altered as sidious, who has probably done far worse.

so, is it using the dark side that results in physical change, or is corruption a direct result of one's own cruelty?

also, is it a cosmetic change or is it force meth? why aren't all sith changed equally?