Hello Sul,
Overall, your setup looks solid to me. Using REFS with a 64k block/cluster size for the 198TB virtual disk is a good choice for storing large files like high-quality videos and images.
Regarding using the 50TB NTFS disk for managing shadow copies for the REFS drive, keep in mind that the 10-20% capacity recommendation for shadow copies is a general guideline, and the actual space required will depend on factors like the rate of change of your data. Make sure to monitor the shadow copy storage usage regularly.