I have both the R7 and the R5 II, and in general I prefer to use the R5 II and an extender to the R7.
Let's start with the geeky stuff. Let's first talk pixels. If you take the R5 II and add the 1.4X TC you of course get a file with 45 MP, now if you crop that image 1.14X it will match field of view of the lens on the R7 (e.g., a 500mm lens will have the reach of an 800mm lens - you can multiply 500 x 1.6 and get 800 or 500 X 1.4 X 1.14 and get 800). How many pixels will be left in the R5 II image if you do this 1.14X crop? You will have 34.4 MP, just slightly more than the 32.5 MP with the R7. That is so close it is really a wash, but the R7 doesn't have an advantage when you do this comparison.
Next let's talk light. The total amount of light hitting the sensor is the brightness of the light, multiplied by the length of time it hits the sensor, multiplied by the area that is being illuminated. For photography we can translate that to aperture, shutter speed, and size of the sensor. It is not true that more light is hitting the sensor when you use the R7 even if you shoot it at a stop faster aperture. Because the FF sensor is bigger even if you use the 1.4X TC, which does make result in light loss and a smaller aperture, you can offset this light loss by turning up the ISO a stop and get the same shutter speed (depth of field will be the same too). So, when you use the FF camera you can just turn the ISO up a stop and use the 1.4X TC and the noise will be about the same because the FF sensor has about a stop less noise. So, light is pretty much a wash too. Just when you add the 1.4X TC know you will have to turn the ISO up a stop, or let the camera do it with Auto ISO.
That leaves us with AF. Now it is true that the TC can affect the AF and slow it down, but there is a pretty big difference between the R5 II and R7 in AF capability. After using both quite a bit, I don't think it is even close the R5 II with a TC has quite a bit better AF than the R7 and that is especially true with high frame rate shooting where the R5 II let's you use the electronic shutter without worries of motion distortion. The R7 really can only use the electronic shutter for still or almost still targets.
So either camera can work, but for me and my shooting I have found that the megapixels are mostly a wash (but a very small advantage in favor of the R5 II if you do an additional 1.14X crop), the light gathering is totally a wash (just turn up the ISO a stop on the R5 II), but the much better AF of the R5 II makes it the camera I always reach for when I want to shoot wildlife or really anything with even moderately fast action.