I have both the RF100-500 and the EF100-400 .... they both work well on the R5

I think with the 1.4 TC you will not notice much drop in sharpness with the EF100-400 but the AF maybe a touch slower.

I have not tried the teleconverters with the RF100-500 but at 500mm it is an f/7.1 lens already, adding a 1.4TC loses one more stop I think.

I use the RF100-500 a lot on my R5 and it is very sharp for a zoom lens, no issue there at all and I am sure you would love it on the R8. (see my Tiger Swallowtail image in the August part II competition) In 1.6 crop mode you sort of get the effect of a teleconverter.

So, for versatility if you plan to still use the 7D2 frequently, go with the EF100-400, RF-EF adapter and the 1.4TC

If you think you are going to go completely to mirrorless (which I think you might once you get the R8!) and sell the 7D2 go with the RF100-500.