I run almost exactly the same thing. Plex running in Proxmox VM with a GPU passthrough, and an OMV instance in Proxmox VM hosting all the data shares. Proxmox also hosts multiple Docker stacks for various instances. This is spread out over multiple bare metal boxes.

I just recently set up a reverse proxy with Nginx Reverse Proxy, and Cloudflare. I pointed my domain to my home address with Cloudflare (they have dynamic DNS capability), then set up NRP, to forward traffic by subdomain. The nice thing about the reverse proxy it is I can bind a subdomain to an ip:port on my local network. Like “music.!MYIP!.com” goes to my Navidrome instance “LOCALIP:4553”. This allows me to close unnecessary outbound ports.

I mainly get my purchased music from Qobuz and Bandcamp. If it's older, don't dismiss compact discs, used or new. A lot of times you can get the CD cheaper than the digital download, then just rip it to any format you want.

I forgot about TIDAL integration, since I'm not a subscriber. I only subscribe to Spotify for the family account. My three teenagers and my wife would all mutiny if I stopped paying for Spotify. I just now convinced them that Plex is stable enough to use for video. Small victories.

Plex server streaming to Plexamp here. Currently handling around 50k tracks all stored on my NAS no problem. Soundiiz supports Plex, so converting Spotify playlists over to Plex is pretty straight forward, provided you have the songs.

• Plex pulls down it's own metadata, so if you're a tagging freak like me, you'll have to check the "Prefer local metadata" on your Plex server.
• Smart playlists are a little cumbersome. They're actually saved filtered searches. Not intuitive at all.
• No HiRes - if that's your thing. (on iOS, not sure about Android)
• Plexamp has a separate EQ for each bluetooth device on iOS, but it can't differentiate between wired headphones using an adapter.
• It does save music or playlists to the device for offline playback, but they're captured within Plexamp. You can't play those offline tracks in any other app (might be possible in Andorid, but the filenames will be random and idk about metadata). I have not run into an offline download limit, like in the old days.