Sticking with what's familiar isn't always the best option. I post/read on forums just so I can find out new things and think outside the box. I think OP posted on this forum probably to get some different point of views than to stick with what he already knows.
I have no problem with adapters but some people do. That's simply personal choice.
My message to OP - Step outside camera brands and figure out which is best tool for your use case.
Does AF matter? Do you think dynamic range, colour depth, tonality are important for landscape?
I would even go as far as suggesting swapping brands to nikon/pentax/sony if you don't like adapting.
If you can't swap overnight you can do it in stages using a cheap adapter in the meantime. There was a A7R going for £725. You can get a A7R+16-35 for under £1500 (less than both canon DSLRs you mentioned).
If you still wish to stick with canon I'd go for 5D4. Good luck