The 16-35mm f/4L IS is one of Canon's best bargains ever, a truly sharp lens with fast AF and excellent IQ. The distortion is something you'd have to deal with at 16mm, but not in every type of photo, and correct composing can reduce the effect. For the price, nothing on the market comes close to the overall quality--including IQ!
Regarding body choice, which seems to have become part of the thread too...I have the 80D and the R. For action, I'd choose the 80D over the R, but for portraits and landscape, I'd go with the R. Specs suggest that AF on the 90D will be significantly better than the 80D you sold. But, full disclosure, I also have a 5DIV, and I'd take that over any of them for action and landscape. Portraits, R for the win!
(I do use the 80D for family fun, for longer hikes where I might get some good macro opportunities--and just to have a robust smaller body with me at all times. I mostly keep the ef-s 35mm f/2.8 IS Macro on it, as it is so versatile, good for portraits, macro, and occasional landscape. And it is so light and tiny, just makes the 80D a convenient, hassle-free camera to have along.)
Maybe save up for a while, patiently hanging in there with your 6D producing beautiful images, to get a better FF body. In my opinion, you can't go wrong with the 16-35 f/4L IS. I've thought several times about getting something faster, but then I just go out and use this svelte little masterpiece, enjoying its low weight, build quality, and IQ. Plus, not having to spend more for an extra stop of light is a good feeling too!
You have interesting choices ahead of you!