Comparing images of full frame against APS-C at same (large) focal length

I looked at your focal length statistics. You seem to use the EF 70-300mm lens almost always at 300mm. If you select a full frame camera without a long lens, your ability for narrow field of view reduces markedly.

Your 12.5 MP calculation is incorrect. The correct formula is to use the square of the crop factor: 20 MP / 1.6^2 = 7.8 MP. You probably won't want to have so little megapixels. While EOS R5 with more megapixels could reduce the megapixel count problem, the lens sharpness may not be adequate for EOS R5, and it's a couple of thousand euros more expensive than the EOS R6, and you can compare the price difference to the cost of a 600mm f/11 lens (the 600mm f/11 costs far less than the price difference).

My recommendation, if deciding to switch to mirrorless, would be the following kit:

• EOS R6 as the camera
• Buy a 24-105mm f/4L lens. You lose slightly focal length range at the long end, but you can use the 70-300mm lens for these use cases.
• Keep using the 70-300mm telephoto for the cases where you need intermediate range
• Buy a 600mm f/11 lens. Yep, f/11 is dim, but full frame f/11 is about equivalent in its light gathering ability to crop sensor f/7.1. Then when you take into account that the 300mm lens is equivalent to 480mm on the crop camera, when photographing a distant subject that won't fill the frame, you're using only 64% of the sensor area that you would be using with the 600mm lens. Thus the 600mm f/11 is equivalent in its light gathering ability to a 300mm f/5.6 lens used on a crop camera, when you crop its field of view to be similar to 600mm f/11. So when photographing e.g. a small bird, you gather as much subject light with 600mm f/11 than you would do with 300mm f/5.6 used on a crop camera.

If you want to save a bit of money, even a 24-105mm f/4-7.1 would be better (gathers more light) than a crop sensor 15-85mm f/3.5-5.6.

With a crop sensor camera you realistically can't achieve more than 400mm focal length with a limited budget (which is 640mm full frame equivalent) and reasonable aperture (teleconverter is a possibility but it has effects on the f-stop), and the Canon's cheapest 400mm option (the f/5.6) isn't stabilized. With RF mount, you have a choice between 600mm f/11 and 800mm f/11.