Honestly I'm blown away right now by the R6 and honestly dumbfounded.
I've owned the EF 50mm f/1.8 STM for years now. I got it when I had my 6DII and used it on the M5 a few times but it was always way too soft wide open for my liking. I even tried using it on the A7III and found t way too soft on that body too so it has always just sat around collecting dust.
I figured that since I have the adapter now, I might as well give it a try so I put it on the R6 and WOW, it's like a completely different lens. The sharpness I'm seeing wide open I was never able to get with any of my other bodies. I honestly have no idea why it performs so much better on the R6 but I will actually use it now as I'm that impressed with it!
The Tamron did look a bit soft around 300mm on the M5 but it's also tack sharp on the R6 at 300mm. I find myself increasing ISO but not shutter speed thanks to how well the IBIS works too.
Well, you have larger pixels now, so 100% pixel views are going to look sharper, even with the same analog focus and stability; or, you may have better focus now and/or IBIS is helping a lot where there was no stabilization in the lens before at all.