there are 2 lenses that i have and kinda could no use (they are for the cropped sensor). 10-18mm canon and sigma 18-35mm (i can use them but only max zoomed).
All the full frame lenses work fine on it with an adapter. i use 85mm a lot and had no problems.
This is my first full frame so maybe i am a bit bias. i do like it. it is not small body but you feel that it is lighter. They did a lot with AF and with stabilization. sometimes you think that R6 20 Megapixel is not enough but there were rare rare occasion where i thought that maybe i would need more to be able to crop. But that is more because my longest lens is 85mm :D
I am not sure but i think you do have 5d IV? if that is the case in your place i would maybe wait a bit to see what new will be out in the next year or so. Especially if you don't do a lot of video.