Can’t get in too deep having never tried anything like this, and because I don’t know OS or RamDisk type.
The Difference Between a tmpfs and ramfs RAM Disk applies to Linux, and claims tmpfs can use swap (so where’s swap living?). Similar question applies to Windows, and if it turns out that the RamDisk holds onto memory persistently, that will leave less for everything else which might then swap or page more to a drive. When the system has a lot of extra memory, the RamDisk will probably be fast but limited by the network…
With Windows systems and Administrator group accounts, –usn-policy might help speed and cut disk load.
Duplicati only uploading changed parts of files on a block-based basis will reduce the amount of data flow.
–tempdir is another option to have less data flow through yous SSD (vendors claim a lot of writes are fine).
Make sure TRIM is enabled.