Wishful thinking?What makes you believe the card(s) are the cause of the lockups? You stated they occurred with other card brands, too.
Actually, I don't really think it is a card problem, but that's the "wisdom" on the internet, and it's the easiest variable to test. But, I haven't found anyone who actually has the problem with either the R5 or the R3 that fixed it by switching cards. Nor have I found anyone who fixed it by doing a factory reset of the settings or reloading the firmware, but I will probably try both of those "solutions" just to eliminate them.
I would send it in to CPS, but I'm in the midst of the busy time of year and can't afford to be without the R3 for even a few days, since I sold the 1DxIII and almost all my EF glass. Plus, I haven't read of anyone who sent either the R5 or R3 in to Canon that had it fixed. I found last night I could simply turn the battery handle to the "remove" position and turn it back and it would reset the camera, so it now falls into the "nuisance" category, rather than the "disaster" category. I intend to keep trying different things and eliminating them one-by-one. Once things slow down a little, I will send it in to Canon, unless it gets significantly worse before then.