I can't help with hardware or configs but some other stuff maybe.
Strictly watching demo's or video's of high level soldiers is generally not what I would recommend at your level of play. Sure it is awesome to watch guys like blaze and seagull make soldier seem easy and do some fantastic things but considering your level, it doesn't help you advance right now IMO. Once you reach a bit higher level of play, those videos will be much more helpful.
Many times in those videos, there are specific reasons why soldiers on those high level teams do what they do and most are likely related to mumble comm's that you don't always hear in the video or the players experience is guiding his actions and he is doing shit automatically. Additionally, playing on a team like that means they trust their teammates to be playing properly and be able to support the action the solider is trying to perform. At anything below invite, you don't find that coordination often so its going to be difficult to emulate right now without the team around you being good enough.
My suggestion would be to work on the bare basics at this point:
--Movement (not just jumping around mindlessly like most soldiers. Work on surfing and strafing as well. Improving your movement is probably the one skill you can increase the most rapidly with just individual practice on your own server. When you jump, are you landing on the flat ground? Try to do jumps that allow you to land on an object instead, like a create or a ledge. Sometimes landing on the enemy is good but often times you should look to land on places that offer you height advantage or make you a more difficult target to hit. Just about every jump you do should have a purpose behind it, be it a better position, a good looking spot to avoid being seen, a safe place to ask for an arrow from your medic, etc, etc. You shouldn't jump around near the enemy without a clear indication of what you intend to do or you risk being mulched up by scouts and spam. Proper movement and control will improve your soldier play significantly)
--Aim (MGE is your friend when you use it properly. Don't play MGE to win. It means nothing to get 20 frags before the other guy. Instead, use it as target practice and start slow. Challenge yourself to try and not miss a single rocket. Fire slowly and make every rocket count. Increase speed gradually as you get your timing and control down. This will benefit you in matches because as you jump in and out of combat, you will have 1-2 rockets and you need to make them count. Control your aim and hit your shots. This skill takes time and practice in match environments so don't expect this one to be quick.)
(IMO, DM is shit for learning to get better. DM is simply raw action that can help you warm-up before scrims but doesn't teach you jack shit. You don't learn to mange your rockets, your health and in most cases your positioning which I think is the most vital for soldiers. I suppose it can possibly help your aim but I believe there are better ways to spend your time)
--Experience (You need to play in match/scrim environments to get the proper experience but there are things you can do on your own to help here as well. You said you have been playing for a while so I assume you know all the relevant maps inside and out? If not, set aside time and learn each map. Look for choke points and the best spam angles for your rockets. High ground should be your life as soldier, so seek out the best perch and think about what you can and can't do from any given position. Know every ammo and health pack location along with every spawn location. You should practice jumps from each spawn location to get back into a team fight as quickly and efficiently as possible. Meaning, when you respawn and have to catch-up to your team, it is often best to jump from healthpack to healthpack until you are back into the fight so that your medic does not need to waste time and heal you. Also work on not taking fall damage from jumps unless necessary or unavoidable. Practice those non-vertical jumps for max distance. Do this for every single map and you should notice improvement in your speed)
Lastly, it sucks to say but often times the team you choose can stifle your progression as well. You seem to have a drive to improve and actually work so try your best to find a team with like minded players so you can learn and help each other. Avoid toxic assholes and teams with multiple anime avatar's as best you can but that might be easier said than done. good luck