Some streaming services like Deezer (free version available) and Apple Music (only paid plans) offer an easy way to add your own tracks your library. I listen to a few songs that aren’t available on Apple Music and all I have to do is drag them into the desktop app and it syncs to all my devices, full with custom cover art and metadata. And iirc, Deezer has a page for uploading mp3s that they then add to your library.
Spotify also has a way to add custom songs, however iirc, it was only possible through individually syncing each and every device you want those songs on with the desktop app.
Never knew deezer has such feature (perhaps due to I use them back in 2020-2021 then ditched it). For apple music not interested with their ecosystem.
The spotify solution require premium subs, besides Im happy to use my self host solution using my old laptop :)
(Currently sitting with 200gigs of mixed MP3 and FLAC collection)