We all have different needs, R5C is a great cam but I personally see very little reason to lean towards it at this particular point in time.
R6 mk3 has IBIS, Clog2, open gate etc. It doesn't have a fan I'll give you that, but tests have shown it won't overheat in the most common 30p modes. If you shoot long form 60p content then maybe there is an argument there.
The cine OS in R5C/C50 is definitely nice, especially for shutter angle as you mention but R6 mk3 has custom modes so just set your high frame rate and shutter speed combos in there or use the new S&F function that conforms to slomo directly (much better imo).
I wouldn't underestimate open gate either, if you have never shot it you don't realise what you're missing. Being able to reframe for social in 2025 is a must and personally I love having the full sensor capture for framing and pulling still grabs.
Clog2 is also a much better log profile than Clog3, less contrasty, more filmic and outputs better DR.
Finally low light will be better on the newer lower MP R6mk3 sensor.