Welcome aboard GCA123.
If you want to compare higher MP camera bodies to cropped sensor bodies in the Canon ecosystem divide the full frame sensor resolution by 1.6^2 ..... ie. 2.56.
So the new Canon R5 at 45MP full frame is equivalent to ~17.5MP APS-C (Canon). So pretty much the same resolution as the Canon 7D II. (It's also equivalent to 20MP APS-C [Nikon] - so pretty much the same resolution as the Nikon D500). Canon seems to have given up on a 7D III. In many ways the R5 is it with extra (profit making) bells and whistles.
I think the most important question is which body focuses most accurately, more of the time, and fastest. Something like the Canon 90D (32MP APS-C) is equivalent to an 83MP full frame sensor - but if the frames are out of focus and/or technique is not rock solid, then a whole lot of softness can result.
The other thing to consider is ISO performance, and there will be times (especially using your lens set up with TC or even bare, at closer ranges) where you won't have to crop a full frame image, or won't have to crop as far as an APS-C. More modern sensors such as the R5 are going to offer an advantage over the 7D II particularly at base, low, and very high ISO's. Check it out over at photonstophotos.
I agree that gains in IQ are likely to be incremental rather than revolutionary. Autofocus performance is the key (whether you shoot mainly static or moving subjects also plays into that).
Chosun :gh: