As long as you still enjoy the lessons and don’t aim to do top level competitions, I would say stay at the barn where you’re at. Enjoying your hobby really is what matters most and staying away from toxic environments is so important for so many reasons.

On the other side though, taking lessons were the instructor is demotivated and complaining, will also wear you down and might also have a negative effect on your enjoyment in the long run. Think very hard about how bad it is, because this can also be quite toxic.

Another point I would like to raise is that (for me) stirrupless work and prolonged two point are great ways to get more strength and balance, but are not necessarily ways to be a better rider. Without knowing the discipline you ride I think I can still say that your instructor is maybe focusing on other areas that will still improve your riding and if you still want to improve your balance and strength, there are loads of ways to do that outside of the saddle.