a good beginner tip is to definitely practice those rollouts, being fast to mid while being at good starting HP means that you can take more pressure so that your medic doesnt tank you hard (reducing other teammates heals) but also do extra early damage to stop early aggression.
on mids be more deliberate with your reloading, because when your team commits you can put out a lot of damage and/or acquire frags
not only on mids but for reloading in general, set up a pattern, b4nny used to do 4 stickies and 2 pipes at a time, as a demo u need to be prepared for anything that might rush or to do damage when ur team needs it most
always be aware of your positioning, ask yourself, am I close to my pocket or am i with atleast with someone on my team, getting caught out alone will hurt your teams defense a lot
practice DM and make sure to take your time with shots, you cant M1+M2 with the nerf in effect
aside from all these tips, pugging, scrims, and matches (so basically gaining exp) will improve your play greatly as demo, every team has different playstyles, know what playstyle your team needs you to play
good luck