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 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).