Final Mod Pack is one of the more difficult mods, or so I'm told. But I haven't actually played it.
UNEXCOM is easier, but may not have the shallowest learning curve, as you noted.
Equal Terms seems to be a fairly low difficulty mod which you may appreciate. It offers plenty of weaponry options right from the start of the game, and they are mostly quite powerful.
I would also like to recommend my own Reaver's Harmony, on Beginner difficulty. There is a lot of stuff offered in Harmony, but the game doesn't force you to engage with most of it. I am also seeking more playtesting specifically on Beginner or Experienced difficulty levels. I have made the difficulty levels vary strongly from one another, so the goal is for Beginner to offer a very casual approach that should not be too stressful even if you are not a veteran X-Com player. I would love some feedback! And if you find it difficult or stressful, let me know because I'll be eager to fix that!