I have owned a 5D Mark II, Mark III, and Mark IV, plus a 7D and a 7D Mark II, and an R5. I believe that you'll notice a considerable improvement between a 7D and a 5D Mark IV. In fact, until the 5DM4, I used my 7D's when I wanted the extra "reach" that a crop gives, but with the 5DM4, I felt that even after cropping I had a better image than the 7DM2. Just remember than none of your EF-S lenses will mount on the 5D.
With my R5, I continue to use some of my EF lenses. The adapters work well. That said, almost all of the native RF lenses are significantly better (and much, much more expensive) than the EF lenses.
What would you be missing with a 5D instead of an R5? For me, the biggest differences include video, which the mirror cameras do much better. Speed - 20fps electronic shutter is amazing, although it's darned annoying in post when you've got so many pictures to sort through. And I don't think you can slow it down, so shooting something like a wedding with electronic shutter (because it is silent) leaves you with an absurd amount of virtually duplicate images. You also don't get the in-body stabilization. In terms of image quality, I feel like the R5 is an incremental improvement over the 5DM4. But to be honest, people obsess too much about the potential IQ of their camera and don't spend enough time learning to best use the tools they have.
I think getting a used 5DM4 for a good price and sticking with the lenses you have is a good plan. It keeps costs down and gives you a significant boost over your existing kit. Alternatively, you could try to pick up a used R6 an adapter and start down the RF route, but that'll be significantly more expensive. The latter becomes much more tempting if you plan to start shooting a lot of video.
Good luck.
