GodOfGreedAs
I believe highschool DXD will be a good place. Not for the women. In DXD you should maybe only make 1 or 2 as his lovers because it's used way too much. The reason I'm saying is because of sacred gears, bloodlines, techniques, skill, abilities, shadow servants, artifacts and many more. DXD is one hella broken and op universe if you ignore the fan service and look deeply into lore. I mean issei even with his plot or luck couldn't have become so strong if it was for draig. So yeah steal, conquer, take all the sacred gears, techniques, shadow servants and artifacts.
I would say Ben 10 but if we talking gathering shadow soldiers how about Dragon Ball world where they have multiple aliens that op race abilities (eg time stop, Ki manipulation, magic, Superhuman strength etc) you can even create a race of your own to slaughter. Dragon Ball World is an excellent pool of exp as the mortals of that world get absurdly strong even fighting gods themselves