Being a flutter dev (and shameless fanboy) I will suggest people try:
appflowy - a FOSS near-clone to Notion.
spotube - FOSS music streaming using the spotify API for metadata and youtube for music playing/downloading. Completely free of ads and works surprisingly well as long as the music you like is mirrored to YouTube.
honourable mentions:
Plex, Nextcloud, Radarr, Sonarr, qbittorrent. Not your usual apps for these kinds of threads but they're absolutely top-tier for linux home servers.
It's not a drop in replacement for spotify, of course. It just uses Spotify's public API for fetching playlists and routes them through Youtube to play the audio of music videos hosted there, but as a free option it's one of my faves :).
If you want podcasts or certain spotify-only remixes you'll still need spotify.