Different guy, but I jumped from GPM to Plex/Tidal back when GPM died. While I mostly listen to my own files via Plex, the Tidal integration and it's playlists are most of what I use for discovery now, and my only complaint is that it doesn't integrate into the Plex/Plexamp apps like the bulk of the library does.
There's a "Daily Discovery" playlist that mixes new (to you) artists + new (to you) tracks from artists already in the library, a "New Arrivals" list for new (to exist) releases from artists you listen to, and ~5 rotating mixes based on artists you've listened to lately. I find the latter to be the closest to GPM's "I'm feeling lucky" though. They're a bit more narrow in scope, but there's 5 of them so there's usually one that goes into the direction I want at the moment.
Having a Plex library, Tidal also opens up options for on-the-fly "radio" stations built into Plex. It has things such as "artist radio" which will play similar artists, which is also an option depending on how much music discovery you're looking to have available.
(Sorry for the long-winded reply, but you've got good opinions elsewhere in this thread and were asking a question I've put entirely too much thought into over the years, so figured I'd aim for thorough).