Hi, I'm the OP.
Yes, the mini split a/c system specs explicitly call for a 50amp breaker and #6 AWG (although I shudder to think that it'll actually use that much, with a 16 SEER).

I think I answered my own question by finding several "conduit fill charts" online that say 1/2" conduit can only have 2 #6 wires, so rules are there to keep me safe and I'm fine with that.

I also failed to notice one last 3/4" knock-out, so I'll be using that one.
Many thanks to all the good technical solutions and suggestions!
Regards,
Theodore.