Building my first 4K editing PC - is it ok?

A couple of caveats to Andy1968's suggestion:

• The advantage applies only with a non-F Intel CPU and having the integrated Intel UHD Graphics force-enabled. If you get an F CPU (which has its integrated graphics permanently disabled at chip manufacturing level due to defects), or if you do not force-enable the integrated Intel UHD Graphics in the BIOS/EFI setup, the advantages would be almost completely wiped out compared to your original choice of a Ryzen 9 5900X.
• I do not recommend a RAID disk at all for backups. In a typical "RAID 0" configuration, there is absolutely no redundancy whatsoever - and worse, if one of the drives fail, the entire contents of the entire RAID 0 array gets permanently lost, and is extremely difficult and extremely expensive to recover, losing years abd years of so-called "backups" forever. You'd be better off using large-capacity single HDDs for storage and backups. (True, all data is "lost" when that single disk fails, but at least the odds of a successful data recovery are much, much better with single disks than with any RAID 0 array.)
• You may also want to upgrade the GPU to a GeForce RTX 3080 for that planned PC. Prices for the 3080 may still be on the high side right now, but at least the $900 to $1000 price for it is much more sane than it had been for many, many months.

Other than these changes, looks OK.

I would go with an Intel Alder Lake CPU. The Quick Sync is worth the price of admission. Some variations of H.264/265 can choke a 12 core CPU but an M1 Apple Chip and Intel Alder Lake CPU can play them back. The video demonstrates Quick Sync and Nvenc in action. It is good to have both 

https://www.youtube.com/watch?v=1L-erwmRxAU

Moved to hardware forum.