You can install and run Stable Diffusion locally (Pinokio is a versatile installer that can run SD and many other open-source AI tools as well). With SD you can build your own upscalers that are better than Upscayl, and do things like background removal too (in addition to prompt-based generation and such).
Another advantage of Nextcloud over Syncthing is selective syncing: Syncthing replicates the entire collection of synced files on each peer, but Nextcloud lets clients sync and unsync subfolders as needed while keeping all the files on the server. That could be useful for OP if they have a terabyte of files to sync but don’t have that much drive space to spare on every client.