Not faulting Canon for trying new directions with RF lenses. It is good to distinguish themselves from other manufacturers like Sony. Unfortunately that is often leaving out the excellent mid-priced lenses often with either very fast, very high quality, but overpriced lenses or cheaper, lower resolution lens. Canon knows the market and what they are doing, but that leaves me stuck in he middle and I would think, a lot of serious but not professional photographers also stuck in the middle. I mean hell, there is the cheap 50 1.8 or 50 1.2. Just gimme a goddam quality 50mm lens like Nikon's 50 1.8. Then there is the issue of the R6. Yes, it is excellent, but for $100 more I could get a Nikon Z6II with 24-70 f/4. Canon has better focus and overall is better, but as one interested in landscape and general nature photography (I don't do video) I could give a rats ass about the better autofocus. I cringe when I see these reviewers running at you and darting from side to side to test the autofocus. To quote John Shaw in his book...John Shaw's Focus on Nature: "Not Once have my lenses gone out and taken a photograph." I don't intend on switching from Canon. It has served me extremely well and I am sure should I go to mirrorless I would still have equipment capable of excellent photos even going with the cheaper lenses, but won't settle for any lens numerically higher than f4, unless something like a 500mm f/5.6, but I won't hold my breath on that one.
If your primary interest is landscape and you aren't concerned about AF, then you should probably be looking at - and comparing - the Canon R, not the R6, to the Nikon Z6. But I understand your thoughts. Earlier this year, I switched to Nikon because their mirrorless cameras seemed a much better deal. The Z5, in fact, would be great choice for anyone on a budget as it has pretty much everything the Z6 does. I bought mine for $899 refurbished. The Nikon Z lenses I bought were fantastic. Dare I say, as goor or even better than similar Canon offerings - but also quite expensive. For the money, if you don't need the high FPS and the more sophisticated AF that Canon offers with their latest mirrorless cameras, Nikon seems a great bargain. Their 14-30mm f/4 is a very good lens, equal in quality and slightly wider than the Canon 16-35 f/4, but why spend over $1000 to switch if you already have the Canon?
Alas, after 25 years or so shooting Canon, I ended up switching back to Canon and did buy an R6 about a month ago. I came back for the Canon color. I know many folks don't notice, or don't care, or might even like the Nikon colors better, but I did not. But if that was not an issue, I would have had no problem saying bye to Canon and going with Nikon for the foreseeable future. They make excellent cameras and lenses as far as I can tell.
One advantage to sticking with Canon , however, would be that you don't need to buy any RF lenses. Since you don't seem to be satisfied with what is offered so far, just use the EF lenses you have now - or even buy additional EF lenses used for relatively inexpensive. If , as you say, you won't buy any lenses slower than f/4, however, I think you are going to find mirrorless lenses in that category are going to be in whatever systems "pro" category and expensive.
