Ask 10 users and you'll get 20 answers...
True syncing is a problems with calibre. Its database is not multi-user compatible, and by "user" that means AI daemons (e.g. file syncing processes, ...) as well.
Many users do fine with most cloud services, but it is essential that you:
- Disable all syncing before launching calibre to do anything. Only re-enable syncing after existing all calibre apps.
Keep an un-synced backup copy of the library
Use a cloud service that accepts any and all filenaming characters and lengths that are supported on the OS you use for calibre. Historically, Google Drive fails this criteria.
I only use two devices so my "syncing" is less of an issue and I find no need for cloud storage. I have calibre installed on both with my primary calibre library on my Windows 10 tablet. I copy this to my desktop periodically to "sync" the two. I do true library management on the tablet only, either using a bluetooth keyboard and mouse or remote controlling it from my desktop using TeamViewer.
When I acquire a new book that needs massaging and/or conversion, I usually to that on my desktop, adding it to the library there. I then do a Save to Disk to export the book (all formats) to a holding folder which is periodically copied to a complimentary folder on my tablet using a local network share. I can then Add the book to my "master" library on the tablet.