Impressive work defeating the 15 waves of the pumpkin moon!
If you're interested in bestiary completion, you could spend the day after very carefully, looking for the halloween exclusive monsters. that can be tough though in hardmode actually, because normal skeletons are quire rare now.
You can also get goodie bags from monsters, and many NPCs sell their costume too, and there are some other unique stuff too.
By the way about power you can acquire in late game, yeah, there are still more powerful stuff you can get.
The next target is probably Golem, and you can get some powerful loot from him too.
Including the picksaw for example, which will technically allows you to mine every sort of block that is in the game - so, that is, the only thing you can't mine yet is lizhard brick, but the picksaw can mine that too. - though it's arguable if it would be the "best" pickaxe, if you ask me, I will often just stick with the pickaxe axe, or if want something very-very fast, and don't mind the shorter range, the shroomite digging claws are quite interesting too. these sort of pickaxes and the like can't mine technically everything, but really, the only difference is lizhard bricks, and, honestly, you don't need to mine that stuff very often in the game XD
On the side of weapons though, well... after golem, the next thing to look for is probably the martians. and they have several weapons they drop - particularly the martian saucers - that are very strong. Influx Weaver is often considered to be a stronger sword than the Terra Blade.
And that's "just" the martians.
In fact, once you're post-Golem, you'll see the cultists conducting their ritual at the dungeon's entrance. they won't mind you until you attack them, but once you attack them, they're pretty weak, and, once all 4 are killed, their leader will appear. The leader of the dungeon coven, the Lunatic Cultist is quite a tough boss too, but with such late-game gear, it's entirelly possible you could defeat him even if you would accidentally started the fight, if you fight well there. And the odd thing about late game is, that events are chained together. At this point, it might be not a very big spoiler, but, defeating the Lunatic Cultist starts the Lunar Apocalypse (AKA Lunar Events) and the various lunar fragments you can get there craft extremely powerful, practically endgame weapons, and this is what make Post-Golem such a tricky thing. Even if the final boss of the game might be still a bit far, it sounds quite tempting to try to tackle the Lunatic Cultist very soon after Golem, to get your hands on some of those late game weapons (there are armors too, and other weapons, but those are all after the final boss, so they're like "post-final-boss" stuff, so there's not much where you actually would need their power as much...
There are still some bosses like Empress of Light, and it certainly would be wise to fight the martians prior to the Lunatic cultist, but it's a good thing (IMO) you actually tried out the moons, and (tier 2) OOA and other stuff first, before going for Golem. I think, many players miss them on first playthrough, because Golem seems very obvious after Plantera, and he's really not all that tough, so you can very quickly find yourself post-golem after post-plantera, and then it's practically impossible to not notice the 4 suspicious figures doing some strange ritual at the dungeon's entrance...
So, post-plantera can easily turn into a boss-rush, and I feel very conflicted about that (even though I myself often fall into that trap as well to do a boss-rush at that point in the game) because this part of the game has so much more to offer than just several late-game bosses. So, I'm glad you tried the moons, OOA and other stuff too.