Let me give my 2 Cents.

a) If you want sharper pictures than I'd rather recommend of upgrading the lense you are using. Just changing the body wont change a lot, if the lense isn't up to par

b) The EOS 90D is a fine cam, that will work well with any current lense. The problem described here is with the 100-400MKI. Let's be clear this is a rather old lense, that generally has issues with softness, no matter the cam. Of course the more megpixels you have the problems will likely increase. But that's the problem of the lense, not particularly the 90D.

c) any resized picture will NOT give you any idea. You'd need to see full (or at least larger) size images for that. At 1.280px resized and edited pic, there is no way you will see any difference between a Canon 1300D and a 1D / 5d, at least with a proper edit. It is at large sizes when you will see a difference.
So yeah those shots are nice, but wont help you....