Hi there, welcome back to the hobby.
Be very careful with those lava rocks. They are very easy to shred Betta's fins now. Most of the fancier Betta's have been over bred to reach the breeders goal and this can make for some very delicate fins. Any sign of fin's getting ripped, I would take those rocks right out. Plus, they can also leach iron which can be potentially harmful as well.
I actually need a little iron for the moss to grow well.
I agree with kanped, no other fish in a 5 with a Betta. It's not that those other fish couldn't be in, it's that there isn't enough place to hide or run away should one or the other be stressed. Betta's tend to be stressed out by schooling fish that buzz around them too much, this can lead to fin biting their own fins out of stress as well.
About a third of the time the fish do not even see each other. As a general rule the Betta has already mad the caves into his undisputed territory.
Your tank has no tall plants for you Betta which they need. Betta's are top dwellers and since your guy has some pretty big fins, he needs places to rest up top.
How tall? Currently he stays in the caves, behind on the pellia moss or rests on top of the central rocks java moss which is about 2 inches thick (and growing, it will grow to the top fairly quickly).
You could keep a snail with him if you wanted or the shrimp as well but no other fish. I would get rid of the Endlers soon before either they become lunch or your Betta becomes nipped. Livebearers in general are known nippers.
Remember Endlers, native type, are not nippers. In fact they are the ones who don't even eat their own young. Not like guppies or other larger live bearers.
Just sharing my thoughts...