Omg hahahahahaha
Your betta need space or it's stressed.
Check the water parameters again, make sure everything is right.
If he needs space, you have to fill this tank with something that will live in a symbiotic relationship. Plants, logs, rocks...not too much surface agitation and they wont appreciate too much light. My betta lives with a 5 hour light time daily, making nests, interacting with the plants and with me, eating a lot, no issues.
U can have some srhimps (same size as the ghost shrimp, any smaller will be food for him) snails, any fish faster than betta will do the work. I guess he didn't kill your tetras cause they are faster.
Try to use nature in your favor, i use 2 aggressive fish with 2 cory, they would kill the cory. However my substrate is black and the cory's colors are dark and dark grey which looks exactly like my substrate, the other fish can't notice them if not by half a inch apart from each other.