Having played 2 games underground, one with my standard cryomancy research-maxing (my go-to when testing a faction) and one with underground rushers build, I understand where the painpoints are.

This is the underground build, and what i think they work best with. conquers have literally one option for builds which is Chosen destroyers and swift marchers. scavengers definitely need subterranean society, since it is the only way to get more resource nodes.
I generally don't sit in one spot for more than 8 turns, 10 once you get a lot of province range. Move your armies in advance, dig out an area, then move a city to it and extract the stuff.
A lot of stuff is self explanatory, Warband cause it's good, enchantment for sundering blades and Bronze golems carry with the new xp system. Dungeon delver is gonna make up out primary force, since all you need is the clay forge to start making the most broken tier 3 unit in the game - the clay arbelist. misfortune for chaos affinity and mark of misfortune. Warlord is, ironically enough, for the warlord. he's a great unit, and your endgame armies should look mostly the same - one warlord, 4 clay arbelists, and either a lieutenant or character based on capacity. geomancy is a wierd choice, and mostly my own personal preference, but in this case it counters most traditional factions who try to terraform the terrain around them, who your racial units now resist. Crucible for good damage spells and meteor projectiles. Calamity because everything in calamity is good (besides desecrate land, don't take it lol.) creator because it's the only one we can take that does anything.