Convert FLAC to MP3
Drop a FLAC and download an MP3 at 192 kbps. The source sample rate and channel count carry through, so a 44.1 kHz stereo album track stays 44.1 kHz stereo.
Encoding from FLAC is the good case: the encoder receives the same samples the original PCM held, so this is a single lossy pass rather than a transcode.
Choosing the bitrate
192 kbps suits most music collections. Material with a lot of cymbal work, applause, or long reverb tails benefits from 256 kbps, which the full Audio Converter exposes along with 320 kbps.
Going below 128 kbps is where MP3 starts to sound obviously thin. If the target device is that constrained, Opus at a low bitrate holds up far better.
Keep the FLAC
Treat the MP3 as a copy for one destination and keep the FLAC as the master. Every new MP3 should come from the FLAC rather than from another MP3, since re-encoding compounds the losses.