Having had enough time to play around with my R6, I've found myself wishing numerous times that I'd bitten the bullet and splurged on the R5.
Like Aether, I end up cropping a lot, and having that extra resolution would go a long way in macro + telephoto. I'm planning to see how prohibitively expensive the 100+ megapixel rumored camera is, and either upgrade to that one or maybe the R5 if it ends up being a 7k+ thing.
On the plus side, with the R6 I don't have to deal with huge files while I'm learning new techniques and shooting 2000+ images per outing, and had enough budget left over for a few lenses which probably wouldn't be the case had I gone with the R5. I like the full frame sensor and the image quality that it offers, especially in low light. To be fair, there's still noise, and you can't shoot in the dark expecting superb images, but I think it's an improvement over what I've seen in the past. The resolution is enough for my needs for the time being. Even after a crop I end up with a large enough image to discern a lot of detail not seen with the naked eye, or use the image as a wallpaper. Haven't tried printing anything.
If the goal is to shoot images that won't be cropped, or for the web, then R6 is more than enough and R5 would likely be an overkill. If you want to print poster-sized pictures, then 20MP might not be enough. A lot of people like the R7, as it has an even higher pixel density than the R5, albeit on a crop sensor - resulting in a smaller resolution overall.
A few things to consider based on wants and needs!