I mean, worst case scenario, you could always try and use the launcher just to download the mods/install and then cut and paste the Minecraft install over to your launcher of choice. That said, I don't know for certain if that works largely because I've never really played any FTB modpacks, and I've always been EXTREMELY picky with Minecraft launchers for a very similar reason (Curseforge's official app has ads and I don't trust that "Overwolf" thing they also make you install to not be spyware data harvesting).
I used to make a lot of use of Newpipe, and probably should get back to using it more, but I've admittedly been preferring to use the mobile site with adblock for a while now, only switching to NewPipe for video downloads, due to how often NewPipe was having issues (presumably caused by frequent Youtube updates). Is that better these days? I sure hope so, it's a lovely app otherwise that I'd love to use more.
As for my own favorite, it's hard for me to pick, because there's several I make use of daily. In no particular order:
I'd also recommend RedReader, but I can't in good faith do that anymore because of what's going on with Reddit these days. Haven't used it or the site much since June.
I'd say my favorite is probably Simple Keyboard just due to how often I use it (read: literally all the time, because it's a keyboard), but I've gotta give mad props to TimeR Machine. It's genuinely a really polished app, and even gives you a quick walkthrough on how to use it on first startup that's pretty helpful. It'd be neat to see more apps with those options.-