The more file-centered, the better IMHO. What Spacedrive envisions is probably my ideal solution.
I use Syncthing for cross platform syncing between Mac/Windows/Linux devices. It’s the most reliable/performant solution for me. But on my phone I actually don’t need sync at all. Basically I’d be happy with on demand file access with thumbnail preview and file management (including deleting files from the Global State without downloading them locally first). Photo back-up (with PhotoKit Background Resource Upload) would be a bonus. I’m fine even with a centralized setup + VPN, just for phone access.
Synctrain has photo-backup but I haven’t tried it as the disk space limitation turns me off. Synctrain has thumbnail support too and can cache them inside a .thumbnails directory inside a shared folder. But it seems my iPhone has to sync the entire .thumbnails directory to be able to benefit from it, which doesn’t fit on the phone. On-demand thumbnail downloading from a (shared) thumbnail folder was being considered.
I’d been hosting Nextcloud on top of the folders synced by Syncthing, just to be able to access them from my phone with a native iOS app. Now I’m transitioning to OpenCloud because it does what I need out of the box without having to install lots of plugins. But if there’s an app which talks directly Syncthing and works more like a virtual file provider (with thumbnail support) I’d happily switch to that. 
Still very interested to follow the development of native Syncthing clients for iOS/Android 