Iron Man, with the DoD contractor CEO hero and the product placement.

I use FRE:AC https://www.freac.org/downloads-mainmenu-33

It can do bulk conversions with a recursive directory search and works in most OSes

I had the exact same use case as you, 1TB of FLACs onto a 256gb phone. Because you prefer minimal quality loss, Opus is the format for you, not MP3. You can maintain transparency-level quality with 128kbps, Opus is roughly equivalent in quality to a mp3 twice its size. AAC and Vorbis are also preferable to MP3 in this aspect, but inferior to Opus. At this point, mp3s are only useful for devices that can’t decode any better codec.

Then i do a search-replace for *.flac -> *.opus on the playlists. I use PowerAmp on android to play the tunes, can recommend.