Well no, you don't really "get it". It's not so much that I "don't like the R", and I totally agree about the horrendous ergonomics of early Sony cameras. I just feel that the R could have been a lot better if it hadn't been rushed out. I did actually consider buying one, but after borrowing one and comparing it to my 5DMkiv I decided it made more sense to keep my DSLRs a while longer, and wait until Canon produced a "better" mirrorless camera - a decision I've never regretted. Ultimately I bought the R5, and I still own the 5DMkiv although it is overdue for replacement with another RF mount body.
To answer your other question "was the 5DMkiv mediocre?" - yes, IMO it was. Just compare it to its closest rivals the Nikon D750, D810 and D850, all of which were launched around the same time (or earlier) than the Canon, but were better specified, better performing and more reasonably priced. I was already too deeply tied into Canon (having previously owned 40D, 50D, 6D, 7D, 7DMkii, 5DMkiii and 5DS, plus several Canon L lenses), and the 5DMkiv had the best sensor that Canon made, but compared to Nikon models it was - mediocre.
In my DSLR days there were many times when, if I could have afforded it, I would have gladly switched to Nikon. I'm glad I stuck with Canon though, as their current models (R6, R5, R3, R7) are all extremely good and serve their respective customer niches very well.