There is a Sonos Support document available here for adding the Plex service to Sonos. Perhaps see this link:

https://support.sonos.com/s/article/3405

Make sure that SMB1 is enabled in your Synology. This will not resolve your track count issue, but at least the first 65,000 tracks can be visible. Note that there is a fixed space allocated in the SONOS players for storing library metadata. If file and track names are long, storage might be exhausted prior to reaching the 65,000 limit.

I have tried Plex, but gave up on it. The Sonos app often can’t connect to the Plex server and, when it does, the interface is slow. Maybe you’d have better luck, but for me it wasn’t workable.

I tried out Plex a while ago using a Plex server on a Synology NAS, and found that the desktop/web version was full featured but the Sonos version was too limited to be any use.

As @buzz mentions, your Synology NAS should work fine as long as you have SMB1 active. As I don’t want SMB1 active on my Synology NAS (which hold all of my real data) I use an LG NAS that’s now been running for 10 years. This has SMB1 enabled, but has no data other than music files on it - it’s effectively sacrificial.

In order to use the Sonos software I have to keep moving music out to make way for the new music. Not ideal, though. I also run an audio server on the Synology NAS (which has SMB1 disabled) which can then cast to  a Chromecast Audio via a line in into a Play 5 Gen 1. This has the benefits that it can handle all of my music and will also natively play high res music (although I only have a few of these files, anyway).

So there are ways of managing larger music collections, but none of them are ideal when working with Sonos kit. As far as I’m aware, the newer S2 kit doesn’t improve matters, either. 

If I recall from the other threads talking about the Synology issue, you also need to turn on NTML v1 (?) Might be worth doing a search on this forum for Synology, and do some reading, just in case.