I have the Canon 5D-MK IV as well as the EOS R. Both use the same 30.3 MP sensor and both are great cameras. The EOS R is lighter in weight and the lenses are lighter and more compact. I like the articulating rear screen. The controls are different from other cameras and there is a higher learning curve. The 5D IV is also a fantastic, feature-rich camera. Many agree that the 5D-IV is the perfect camera but I wish it had an articulating rear screen. The 5D control system is extensive and amazing. It is heavy compared to the EOS R. Keep in mind that Canon is beginning to discontinue the EF lenses that are compatible with their full frame DSLR cameras. The EF lenses can be used on the mirrorless products with adapters but the RF lenses for mirrorless cameras cannot be used with EF mount bodies. That is a signal that the canon full frame DSLR cameras will not be manufactured in the near future. For that reason I would advise investing in mirrorless models with the RF mounts to avoid future obsolescence. At present, the RF lens line is limited but Canon has announced numerous additions in te near future.

Between the two, I would lean toward the 5DIV for it's more sophisticated AF system and higher resolution (30MP vs 26MP). The 6DII might be better for high ISO low light shooting, though maybe not by much.

However, define "nature photography". If you are talking about wide angle landscape shots and macro photography, a full frame camera like the 5DIV might be great. But if you mean wildlife.... especially smaller critters... you might be better sticking with one of the APS-C cameras... perhaps a 90D (which actually has higher resolution than 5DIV and an updated version of the 6DII's AF system).

Look at it this way.... for wildlife shots you take now with a 200mm, with full frame you'll need a 300mm. And for what you shoot now with 300mm, you'll need 500mm. Or if you have a 500mm now, you'll need an 800mm on the full frame camera. Look carefully at the sizes, weights and prices of the lenses you'll need! Unless you're planning to make prints larger than 16x20", you might want to stick with a crop sensor camera!

For street photography I wouldn't want either 5DIV or 6DII. In fact, for street (and some specialized candid portraiture) I recently bought a Canon M5 mirrorless camera (APS-C) and several compact lenses (12mm, 22mm, 56mm, 90mm). One of the lenses I expect to use most is a Canon EF-M 22mm f/2, which weighs under 1/4 lb. and is almost a "pancake" (I've fitted it with a 3rd party lens hood that nearly doubles it's size!). The entire kit of M5 and lenses weighs about the same as one of my 7DII and moderate size zooms. To me a "street" camera needs to be small and unobtrusive. I want to look like a tourist, not a pro. This can work well for candid portraiture, too.

If I really, really wanted full frame for street photography, I'd have to look at some of the most compact mirrorless such as Sony a7C, Sigma fp and (once I win the lottery) Leica M-10. I'd have to look into what compact lenses are available in each system though, before deciding. A Canon RP isn't quite as compact as those other cameras, but might serve well too, perhaps with RF 35mm f/1.8 and RF 85mm f/2 lenses.

Just note that any Canon EF-S lenses you have won't mount on either body, and other brand ones will vignette unless put in the APS-C mode which crops the image way down so less than 1/2 the pixels are used.

While many swear by full frame, I have no major complaints with my 80D. Going to a 90D would allow you to use all the lenses you have for the 7D. You haven't explained why you want to replace it. Of course, you save the most if you stay with the 7D

Just note that any Canon EF-S lenses you have won't mount on either body, and other brand ones will vignette unless put in the APS-C mode which crops the image way down so less than 1/2 the pixels are used.

While many swear by full frame, I have no major complaints with my 80D. Going to a 90D would allow you to use all the lenses you have for the 7D. You haven't explained why you want to replace it. Of course, you save the most if you stay with the 7D