Luke Chmilenko‘s Starbreaker series starts off as a fantasy novel with minor romance elements before fairly quickly dropping almost all of the latter to shift over to magic-infused sci-fi. It mainly stands out for how the military organization the protagonist joins ends up having harsher training methods than the evil apocalypse cult he grew up in, and I had to drop it in the second novel when his instructors decided that deliberately inducing dissociative identity disorder via physical beating was preferable to simply allowing him to modify his spells.
I’d been avoiding Benjamin Medrano‘s novels after being disappointed with the abrupt developments partway through Through the Fire, and that turns out to have been a mistake as his Eve of Destruction series is pretty much exactly what I look for in overpowered protagonist series. Main thing to be aware of going in here is that, with the notable exception of the third novel (whose second half features dates with each of the harem members), the romance elements are quite minimal.