I’m not familiar with Sync.com, it does look interesting with end to end encryption built right in.
Does Sync.com use a local Sync folder? If so you can do a FILE SYSTEM Sync and place that sync folder within the Sync.com’s sync folder. Then point all your clients to the copy of the Joplin sync. If it’s like other sync tools it usually does not offer to actually sync files on mobile devices, it just allows access. So that would be a short coming.