You can go down to 18" rims but you won't find aggressive tires like you might find for a Jeep as an example. You can always put on different rims that will allow for the proper offset and bolt pattern. The F Pace is not really meant for off roading although some may believe otherwise.
I’m with you, I just like to make mine a little different. I’m gonna stick with the 20in rim although I know they takes some options away. I bought a set of 285/50r20 Nitto ridge grapplers today. I’ll see if I can make them fit tomorrow morning.
I think a 20" rim and the 285/50 size probably won't clear unless you really push them out away from the spindle with some generous offset The key is going to be the rim width and the ET offset of the wheels. I can tell you that I had a 285/35 22 inch tire on 22x10.5 inch rims at ET of 40 on 2019 SVR that I just totaled back at the end of March, that tire setup on that wheel just cleared the inside spindle and I think your challenge may be there as well and probably will hit the spindle due to the height those 285/50 tires add to the equation. At @ 790 mm high, they are some 40mm taller than the 285/45s tires I had on my former SVR. So on the top end on the inside with the spindle you'll have pretty much no room for error.
I know with that car the top of the tire just cleared the top turn of the spindle. It was so close I could not get away with a 295 tire or even some 285/35s from different manufacturers.