I jumped from a pair of 5DIII's to a R6mkII a few years back and the difference in camera was like night and day. Better high iso ability, way more optical resolution for a similar sensor resolution, faster fps, better metering. The AF was on a completely different level, I had a lot more dynamic range to play with. It's like every metric was vastly superior. Things had really moved on well beyond just MP ratings.
I found the EVF had a lot of benefits (like low light and I could now see into the sun without fear of retina damage). Histograms, electronic levels in the view finder....so much better. Although a good optical view finder is a joy...they were becoming quite rare.
Now that I have a R5, I came quite late to that party....i'm finding the age difference between the R6ii and R5 to be irksome. The AF is weaker on the R5 and there's a lot of other improvements on the newer body that make life so much nicer. I can only imagine what the R5ii or R6iii are like in that regard.
So here in early Jan 2026, I'm now telling myself to save for a R5ii to replace my R5 mkI later in the year and then focus on glass. I got such a good deal on my R5, I'll probably get close to what I paid for it anyhow.