I've regularly bottomed (vers) on and off for over a decade. The hole visually changes with regular use over years (not including size training or gaping) but not in any dramatic way. For most bottoms a hole with average experience might look more wrinkled or darker but not like a slit. You have to train for that kind of slit look.
It's like a muscle for most people, so with experience it gets more toned if it's just regular dick in ass or small to medium toys. I find if I'm regularly bottoming I seem to have less issues with my hole from daily life like micro-tears. No continence issues.
Definitely more refined sensation with use and sensitivity to stimulation, better ability to discern and the pleasure has gotten significantly better, more complex. This is the biggest change. Went from uncomfortable feeling, to nothing, to erogenous zone, to additional complimentary pleasure and nice fullness, to having its own craving itself, to actively stimulating for pleasure, to prostate orgasms/milking, to anal orgasms.
On looseness, it's more skill than anything and the hole begins losing skill to open up without regular use (more than once a week). Give it a month or month and a half and I may have issues taking anything even teensy tiny (bigger than a pinky) without active work. The skill and ability to bottom seems to be half-muscle memory half active skill. I find that there's a set point where your body adapts, that takes years to get to but eventually your body can take a certain set size and everything below it is easy-peasy, not even registering on the radar of difficulty. This has changed twice in my life, for context. This isn't looseness because if you take a break you need to still re-familiarize with regular opening up, but the benchmark does return.
Half-muscle memory because it's very clearly easy to upskill when you return to bottoming. If you train to a certain length, it's hard the first time you do it but every time in the future will be easier if you take a break.
Even after all these years I have the occasional tops blaming me for tightness, when the tightness is them not allowing me the time to open up due to impatience. It's extremely frustrating and concerns me about our culture right now. Even with experience, it's something that requires active relaxation to open and properly as well as safely enjoy - or opening with a toy before even meeting with a top.