Hi and welcome to the community!
What would be the best Raid type for these drives? Splitting them up into 3 Raid 10 for each backup above?
If you do not plan to switch from Reverse Incremental to Forward then I'd recommend you to stay away from RAIDs with parity (5 or 6), RAID 10 is good to go with. Splitting will eliminate all performance benefits of having many spindles, so it is better to stay with a single array. This article
is also useful.
Please also check this
Also as the current nas mounting solution is using iscsi on to the veeam server, does this provide any draw backs / performance problems?
iSCSI is fine. Since you plan to use 10 Gbit, the link is not likely to be a bottleneck in your case.
Also the large VM is very slow when performing FLR, I presume this is due to A: the Size of the VM and the server its sitting on with the 1GB links?
Which part of FLR process is the slowest? Browsing for files to restore or the actual restore process?