It's no good hentai if it doesn't get hard, right?
This is my game and I'm all for the fluffly and cute consensual thing. This time at least.
I thought Jomon was in, if not I'll pick whatever is left because I don't have any UW-experience and therefore no UW-preference.
Jomon has
quite strong UW options, but the problem is to get UW first of all, then expand properly. I won't stop you, but consider that you are already getting somewhat of a malus. Caveat Emptor!
Regarding UW powers, they can be somewhat explained this way:
+ Atlantis: first two ages are Ogre nations with good troops in EA and terrible troops in MA. Basalt Kings are some of the strongest mages in the setting if you know how to properly employ them, and their troops are amphibious and flexible. They are kind of the middle of the road, as they can live and die UW and aboveland, but they excel at neither. They sorely lack research monkeys (cheap mages for research). LA Atlantis is another beast, and it has adequate mages, awesome infantry and flexible UW recruits.
+ Starspawns: I have no idea how EA Starspawns play. Good UW mages? MA Starspawns have adequate troops (some are good at fighting, some are THE cheapest recruitable mindbroken chaff par excellence), and a good amount of Astral magic and Starspawn shenanigans. LA is the same, plus insanity and domkill and freespawn you gotta pay. LA Starspawn is not an excellent choice.
+ Pelagia: Pelagia is triumph UW, suffer somewhat aboveland. MA Pelagia is strong as fuck, having good mages, excellent UW sacreds AND workable aboveland sacreds. Never played EA Pelagia.
+ Oceania: Pangaea UW. Strong UW (not as Pelagia though) but I know little. Never played 'em.
+ Therodos: Therodos is not a UW nation. They are subpar at being one.
+ Ys: heard their sacreds are good. UW elves? What's not to love?
Honestly, I'd suggest to run some test games and see what you like. Jomon is truly a expert-level pick.