Helpful advice so far. One thing hasn’t been mentioned: don’t swing, I mean, don’t backswing on serve returns, especially if they are coming in fast. They already have the speed. You just need to “deflect” it back. So if the serve is going to hit the first glass, I take a step back, prepare my racket low and in front of me and wait for the rebound. Stay on your toes and watch the ball. You can always go towards it (but not backwards). So if your racket is prepared in front of you and low, and you are watching the ball rebound, then you basically just “pick up” the ball - you have two options, either try to hit a lob return, or hit it shoulder/head high towards the backhand side of the server. Try both return directions. You’ll learn what works for you and what works against other opponents. (I’m 2.6 Playtomic level in Spain)
Edit - just to add, shoulder / head high backhand area is good to return to, because usually the server will be running towards the net, and a high backhand while running is a hard shot to hit, they can’t do much with it. If it’s a high forehand, they can smash or hit an ok volley, but a high backhand is an awkward shot for beginner / intermediate level, especially while running