Hi, Deborah. I'm no expert, but my experience is that syncing my Mac to both iCloud and Dropbox is unreliable and uses up too much space on my local drive.
Dropbox support told me not all Mac files will save properly to Dropbox because they're in "packages".
Apple support told me not all Mac files will save to iCloud because the macOS insists on keeping some things local.
What you end up with is fragmented files.
My solution has been to keep current projects local and backup to Time Machine continuously. I also save projects to an external SSD drive weekly.
I save files to iCloud if I want to use them on different devices connected to iCloud. AirDrop is useful, too.
I use Dropbox as an archive, only.
Sometimes files become corrupted. Time machine and external backups have saved me from losing work and time.
There is really useful support buried deep in the AVID Sibelius knowledge base, but you have to be patient and burrow long enough.
Good luck.