I cut my tall acer back a few years ago and now it just produces long thin stems, sometimes a few feel in length. I keep cutting them off and it stays small but not a nice shape.
I don't know what can be done about it.

I'm not an Acer expert but I can't see any problem with pruning the long branches back to original shape. I would do it now but make sure that you prune just above a leaf node so you don't cause any die-back.

Welcome to GoY.

I would trim the long ones back to just shorter than the branches that are in the right place. Then when the new side shoots grow they shouldn't be as long as the rest of the canopy.

We have several acers, all in planted in the ground. OH prunes them in winter, he doesn't prune in summer as this will promote growth which is unpredictable.
Best waiting til they've stopped growing, then prune to the shape you desire.
Prune it now, today even, to establish the shape you want for this year.

I had this same issue. I simply pruned them all back to a decent shape and that seemed to work. I think I did my pruning in winter. The next year I got far less long straggly shoots and I did the same again. Mine was A er Sango Kaku, the Coral Bark Maple. MY other Acers didn’t do it as much. I read upon it and found the general advice was it was a result of pruning too severely, but that wasn’t the case as I had only ever pruned out dead matter until then. So y advice is just cut them back during dormancy and don’t worry.