The R, EF, EF-S, and EF-M are all different takes on just about the same mount. They all lock lenses on the same way and have the same basic communications protocol between lens and body, their primary differences come down to lens coverage (full frame vs crop) and flange distance (to account for the mirror box). Adapters from R to EF and EF-M to EF/EF-S are widely available.
With a full frame camera such as the 6D Mk II or EOS R, you get two big advantages:
1) Another stop of low light performance and shallower depth of field (an f/4 lens on a full frame is equivalent to an f/2.8 on a crop sensor). This also means that with the same f/2.8 lens, the full frame body will have 1/2 the high ISO noise (ISO3200 on a crop sensor looks about like ISO6400 on a full frame).
2) Wide angle lenses (in terms of the same field of view) can be made more easily and more cheaply all else being equal, especially on the long flange DSLR mounts
And that's about it. Some FF cameras have higher megapixel counts, but those two really don't by enough to be meaningful. They are both highly capable cameras, but are those going to be an advantage to you?
The DSLR mounts do have much deeper and broader selections of lenses, but that has more to do with the age of the mount. Nikon, in particular, has somewhere above 500 current and past first party lenses that work fine on their better cameras, and Canon has about 200-ish, and that's not counting third parties with thousands of lenses, especially for the F mount dating back to the 1960's. That advantage isn't negligible, but it's also not something that should affect you at SWGE. Using a 180 degree circular fisheye with perfectly corrected astigmatism and coma designed for Antarctic astrophotography just isn't a useful capability for 99% of shooters.
I have both a full frame and a crop body, and the crop body definitely gets used more, even with somewhere like SWGE. I'd pack my crop body, 12-24 f/4, 16-80 f/2.8-4, a fast prime, and some sort of telephoto (probably my 105 micro), and not worry about it. If I can't get the shot with those lenses, I would need something highly specialized and the learning curve on those is tremendous: realistically, I'd need to purchase the lens in order to get proficient enough to take photos with it that are better than I can get already with my existing lenses and software corrections (you can perspective control non-PC lenses in software, with some loss in resolution).
And I have over two dozen lenses in my stable: an 11-22 that you know how to use is going to give far superior results to a full frame body with 17mm tilt shift that you're not familiar with.
If you want to rent, you would be better off renting to get a faster or better lens in a focal length range you already know how to use on your existing body than to rent an entirely new from scratch system. A 70-200 f/2.8 is a great candidate for that, or an f/2.8 wide angle when you already own the slow one.