If you want to use an EF-S lens, you cannot do so with a FF DSLR - they physically do not fit and there is no adapter. Leaving that aside, there are lots of FF DSLRs that might work for you. The 6DMkII is a great camera (as Rick has said).
As far a the R-series Mirrorless Interchangeable Lens Camera (MILC) bodies are concerned, there are several options here. The current top-tier body is the R5, then the R6 and R6MkII, and then the R8. The R7 is an APS-C camera so does not fit your FF requirement. The first three are prosumer bodies with the most features, but the R8 is a great camera, with the same sensor as the R6MkII, and a lot of other features, for a significantly lower price. Of course there are compromises to get to that price point, but they may be ones you are happy to make. See this video, which does a comparison of the three:
With regard to your lenses. EF and EF-S lenses can be made to work with the R-series MILC bodies, via the EF-RF adapter, which works seamlessly. There are some compromises however. You will not get the integration of the Optical Image Stabilization (OIS) of the lenses and the In-Body Image Stabilization (IBIS) of the R5 and R6 bodies. The R8 does not have IBIS so that is not an issue.
Another issue is that if you put your EF-S lenses onto the any of the FF R-series MILC bodies, they will work but they will automatically put the sensors of the bodies into APS-C crop mode to match the Field of Capture (FoC) of the crop-sensor bodies they were designed for. This will significantly reduce your pixel count by a factor of 2.56: so the R5 drops from 45MP to 17MP, the R6 from 20MP to 8MP, and the R6II and R8 drop from 24MP to about 9MP. Whether this is significant is something you need to decide for yourself.
As regards the image quality of EF and EF-S lenses on the R-series FF bodies. There is no doubt that native RF and RF-S lenses do have advantages in design, and some lenses are definitely noticeably ahead of the equivalent EF and EF-S units. But as my respected colleagues state, and I too have found, in many cases this is not significant. I would say that the older and/or cheaper the DSLR lens the more it is likely to be a risk.
Without doubt, the DSLR and M-series MILCs are living on borrowed time and are showing no further signs of development. Supply of current models will continue as long as Canon sees a viable market, but it is clear that they are replacing the old gear with compelling new lenses and bodies, with many more to come. All that said, if you prefer (as you seem to suggest) to stick to a DSLR body, there are bargains to be had as folks move to the MILC platform.
cheers, TREVOR
"The Amount of Misery expands to fill the space available"
"All the variety, all the charm, all the beauty of life is made up of light and shadow", Leo Tolstoy;
"Skill in photography is acquired by practice and not by purchase" Percy W. Harris
