Electronic or mechanical shutter on R5 ?

Yes, it is my experience too that on the EOS R5, the MS causes shuttershock and slightly unsharp images. I tested it with RF 100-500, Canon EF 85mm 1.4 and Tamron 35mm 1.4.

It was really bad at 1/80, 1/100, 1/125 and 1/160. Really bad means you can see the difference when comparing to EFCS, so it's "relative."

It was OK at 1/50.  At 1/60 and 1/200 it was still OK but EFCS was better. Even at 1/400, EFCS produces a slightly sharper image than MS.

Which shutter mode to use? Well, I come from FUJI X-H1 and there was a convenient setting:

• EFCS until 1/2000
• MS faster than 1/2000
• ES faster than 1/8000 (not applicable for the R5 of course)

Unfortunately, the geniuses at Canon can/do not offer this on the R5 (???).

I tend to use EFCS most of the time. The IQ seems to be the highest. There are 2 disadvantages I can live with: the first picture taken after turning the camera on, is blurry. Always. So I snap a quick shot without even framing, knowing it's a wasted shot, the rest will be sharp.  And supposedly - I have not tested this myself but seen it on YouTube, at speeds higher than 1/2000 and a wide aperture like 1.4 or 2, the bokeh can be affected. In that situation, switch to MS or ES.

In case you experience banding or rolling shutter with EFCS, switch to MS.

When to use ES? If you need 20 fps, or silence.  For wildlife and landscape you can use ES as long as the object is not moving fast and there is no artificial light in the scene.

EFCS and MS give you a 14bits image, ES a 12 bits. In case that matters to you.