There hasn't been a Subsonic update in nearly two years, so I don't think you'll have any luck getting any support. That said, have you tried Airsonic? It's a fork of Subsonic with the fee requirements stripped out. It's since been forked to Airsonic Advance, and while I couldn't find a Synology package for that, there's a specific build of Airsonic for Synology:
https://github.com/ghenry22/airsonic-sy ... g/1_0_1062I switched to a NAS last year and kind of saw the writing on the wall with the *sonic releases, so I took a look at Koel, Navidrome, etc., and ultimately settled on Emby, as I was already using it for video. It still not as good as Subsonic was for my music needs—it's first and foremost a video streamer—but it is under active development and getting better with each release.