as far as projectile classes go its all about getting in the reps. soldier and demo are pretty lenient on mouse control you just need to build up the intuitive understanding of how people move and how your projectiles work.
IMO for both classes the best way to get better is:
pugging > dm > mge.
people move very unnaturally in mge because there's no objective other than winning a fight, in a real game you can use your understanding of the other players win conditions and objective to hit them more. dm kind of mimics this by people having to worry about more than one person but pugging is definitely the best practice
conversely, if your someone who wants to get good at tf2 as soon as possible just grind the fuck out of scout dm/mge cuz u can have awful fundamentals and still play at a div 1 or 2 level just by having good aim and playing the beam
some of the most meteoric rises ive seen in tf2 have just been scouts with a 2k hours of mge completely steamrolling a lower division in their first season and getting on teams off of that