I find FOWLR to be easier than a lot of freshwater tanksPersonally think the height would be perfect for the altums idea with some smaller supporting cast. Could be a blackwater biotope with some plants, planted does not have to be high maintenance! Plenty of easy low-light plants which basically take care of themselves. Could just fill the tank with valisneria and maybe some java moss. Those two require little to no care and will fill the tank up for a naturalistic overgrown look. Along with some wood and sand/pebbles, could be an easy beautiful tank IMO.
Single big cichlid would be cool as well of course. Personally I have become less fond of this sort of setup over the years because I enjoy making the tank it's own enclosed community and watching the inhabitants interact. But that's my personal preference.
And last but not least, saltwater would be awesome! Could do a fish only with live rock (no corals) for easy maintenance and have interesting species. Lionfish tank would be cool! Only real difference from fresh to salt is that you have to mix the salt into the water for water changes, which is pretty easy. Reef is where it gets complicated.
Good luck, let us know what you pick! Cool tank size