No, sorry I haven’t tried it with Syncthing. Mainly using it for immich, seafile, a matrix server, some arr apps and a status monitor called dashdot. Would be useful for syncthing though, never thought of trying it - I’ll give it a shot over the weekend and let you know how it goes!
If you’re planning on upgrading the CPU, GPU and RAM, have you considered keeping the old stuff and running 2 systems, splitting the load so to say? Could do those random services and Nextcloud, photoprism (unsolicited suggestion - try Immich too!), the OCR thing, etc on the old one and keep the new one for gaming, your forum that needs transcoding, and Jellyfin?
Edit - Afterthought: your old system could then be headless too