Beginner camera recommendations

I'm looking at getting into photography and I'm trying to decide on what camera would be best for me. One of the primary things I'm interested in is nature photography when I'm out on hikes.

I spoke with someone I know into photography and he seemed to be pushing me towards full frame cameras. Given the prices for these cameras (I'm fine with used) it seems like I have to make a bit of a choice between older full frame and newer aps-c.

After doing some research here are the cameras I am currently considering:

Sony a7

Sony a7R II

Sony a6400 (or maybe a6500)

Nikon D800

Canon EOS 5D Mk III

Canon M50 Mk II

These are sort of organized by how interested in them I am. The pricing varies a bit for them, but with the cheaper ones I'd be able to get more lenses sooner and/or feel less bad about upgrading it in 2 years if I get really into it. So would the a7 or a7r be a good way to go to get a good balance of things or is something like a D800 still worth it today? Or is it just that worth it to have something like the a6400/6500 today to have newer features?