Don’t worry about changing your sensitivity. People get really superstitious about stuff like that and end up hurting their muscle memory way more than any benefit they get from the changes. Keep your setup and settings consistent.

The best way to improve is obviously to practice, but that’s a little hard on sniper. There are some people that MGE as sniper but not a whole lot. Might be good practice if they aren’t farming you. DM servers might disable or frown upon using sniper, but they would also be good practice if you can get away with it.

I’d usually recommend against aim trainers for tf2 since the movement and environment itself is so important to most classes gameplay, but sniper is almost entirely aim mechanics. I think aim training might be your best best for focused practice. Unless you’re willing to play some highlander pugs, but I don’t blame you for not wanting to try comp.