EF lenses adapted for use on R-series cameras work just as well and sometimes even better than they did on the DSLRs for which they were originally intended.
If you get adapters, there are some lower cost 3rd party, simple ones that are inexpensive and will work. But if you want the "control ring" type, you should buy the Canon EF to RF with that feature (cost is approx. $199).
In case you are unaware, many of the RF lenses have a built-in control ring that can be programmed to serve different functions. You can use it to set lens aperture or to change ISO or shutter speed, or to adjust Exposure Compensation, etc. Or you can turn it off so it does nothing at all.
EF lenses don't have that control ring. So there is the Canon version of the adapter that has it. Other brands of control ring adapter may not work properly (I've just read about one of those on another forum).
There also is a cheaper Canon EF to RF adapter ($99) without the control ring. That's a simple adapter with electronic contacts that "pass through" communication between the camera and lens.
There also are more expensive EF to RF adapters with drop-in circular polarizing or variable neutral density filters. You probably don't need either of those because you can add filters to the front of your lenses easily, if wanted.
Among your lenses, the EF 100mm Macro and 24-105mm are worth adapting for use on an R6. The 11-20mm and 18-400mm are not. Those are designed for use on an APS-C camera, not full frame camera like the R6 (or, for that matter, the full frame 5D Mark IV).
However, you may want to consider the R7 or R10. Both of those are APS-C cameras that can utilize all your lenses via an adapter. The R10 is 24MP, while the R7 is a 32.5MP. Both share similar AF system with the R3, R5 and R6 full frame cameras. The $1499 R7 has in body image stabilization (IBIS), the R10 doesn't. The R7 has dual memory card slots and some level of weather resistance. The $979 R10 has minimal weather resistance and single memory card slot. The R7 uses a larger battery to get nearly 2X as many shots per charge as the R10. The R10 has a built-in/pop up flash, the R7 doesn't (which may be part of the reason the R7 is better weather sealed). The R7's shutter can be set to close and protect the sensor whenever the lens is removed, as can the R6's. The R10 doesn't have this feature.
The full frame R6 with its modest resolution will be a better camera for low light photography, where higher ISOs are needed. But in many other respects the APS-C R7 is its equal or may be a better choice.
You mention photographing animals... I am guessing you mean wildlife and/or pets. If you use powerful telephotos for that, the R7 APS-C camera can be an advantage. It makes a 400mm lens like yours "act like 640mm" would on the full frame R6. In other words, to enjoy the same "telephoto reach" you have now or with the R7, with the full frame camera you will need a much more powerful telephoto lens. That will be bigger and heavier... and probably more expensive.
Seeing that you currently have two lenses designed for APS-C cameras, I assume that's what you are using. If that's the case, then all your lenses would perform very much the same, with angle of view much like you have now, adapted onto the R7 or R10. That won't be the case adapted for use on full frame cameras. Again, only your 100mm macro and 24-105mm will be practical for adapted use on full frame.
If you can, go to a store and handle these cameras. There's a difference in the control layouts, grips, etc. that might be important to you. The only way to tell is to actually handle the cameras.
Have fun shopping!
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7DII (x2), 7D(x2) some other cameras, various lenses & accessories
FLICKR & ZENFOLIO
