ffmpeg

ffmpeg -i in.flac -c:a libopus -b 192k out.opus

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.

I recommend converting to opus. Best efficiency.

dont convert them to mp3, use either AAC or Opus, 192kbps is typically good enough for high quality, but a lot of people will just encode 128kbps

I dunno, I normally use my bash script + ffmpeg to convert batch flacs to mp3s

#!/bin/bash

cd "${1}"

for subdir in *; do
    cd "${subdir}"

    for input in *.flac; do
        echo ${input%.*}
        ffmpeg -i "${input}" -ab 320k  -map_metadata 0 -id3v2_version 3 "${input%.*}.mp3" && rm "${input}"
    done

    cd ..
done

Then i'd just run my script.sh [directory that contains flac] you might want to remove && rm "${input}" if you don't want it to delete your flac files automatically.

Everyone is recommending ffmpeg. So am I, but with Axiom UI, since everyone is not used to working in terminal. You can convert entire folders just as well like you could with ffmpeg normally. I am used to command line stuff, but I still prefer this for simple conversions out of convenience. https://axiomui.github.io/

dBpoweramp

I use FFmpeg Batch Audio Video converter, it's just a GUI FFmpeg. I use it to convert FLAC to MP3 when I was going on a trip and only had a CD Player and I use it to convert 7.1 Audio down to AC3 5.1 to fit my sound system. It has a MP3 preset but I had to add one for the 5.1 conversion.

I used to use musicbee for this. Good app for organising your music on your desktop and you can configure it to convert audio format when syncing to your phone. It's been a while since I used it, but it did the job nicely

Audacity free and open source also user friendly

If you're using Windows then Foobar 2000 is great. I use it for manually managing and tidying my collection in addition to converting pretty much any format to any other format.

That being said, you can install it as a snap in Linux as well if that's your poison.

I personally use AIMP. It has a built in converter it worked well for me

Besides ffmpeg I would also recommend audio converter for the ones who want an Android app with a GUI, I believe it uses FFmpeg under the hood

What have you set your compression level to for the flac files?