This is one advantage of keeping/buying EF 70-200 or 100-400 lenses for the R cameras. You can use an EF-RF + EF 1.4x / 2x TC III + 70-200 or 100-400 II with the R bodies. I guess I should give this a test tomorrow to make sure it works.
You'll be fine. I used 70-200 f/2.8 IS III > 2.0x extender II > 1.4x extender II > control ring adapter in the R (not the R5 yet), and was able to autofocus no problem. I would LOVE to replace all 3 with the RF equivalents but it's just not in the cards. I wish they'd make a reasonably sized RF zoom that starts at 200mm or so, then I could carry 2 lenses instead of the TC's... The 100-500mm doesn't go as long as I would like and 100mm overlaps too far.