It’s worth keeping everything archived somewhere in FLAC if you care about it, because that’s lossless and you can convert it to newer cooler formats in the future. Opus is currently the best lossy audio codec - if you need space on your phone, convert an additional ~128kbps Opus copy of your library for transparent quality and super small filesize.
Don’t believe anyone trying to sell you on the idea that FLAC sounds better than an appropriately compressed (read: transparent) lossy format: Opus ~128-160kbps, MP3 ‘V0’ (~215kbps), MP3 320kbps, AAC ~150kbps.
Check the “Music encoding quality” table on this page for more info on Opus bitrates and how they relate to transparency.