I’ve been having almost the exact same issues since 8.3.7. I have tried a clean install of WoW and doing the fixes/changes found in Known Technical Support Issues - Updated July 21, 2020 - #16 by Caterpepi. These have only lessened the issue, the freezing still happens.
I can recreate the issue occasionally by opening up an interface menu like transmogs or pets but it happens at unpredictable times and causes 3-6 seconds of freezing. I’ve watched my GPU jump up to 100% momentarily then drop to almost nothing until the freezing stops. When the freezing stops the screen will flash and it looks as if all the graphics are re-loading. I am providing a dxdiag pastebin.
Edit: This happens a lot more often in the Great Seal.
http s://pastebin.com/X7iufyTv