Thank you all above for your advice.
You shift me towards an early adoptation of the R5/6.
All three of you seem to point to - which was my underlying concern - a solution based solely on R-system being preferable compared to mixing EF and R-system.
Especially when the timing is as it is. (I guess I shold have invested in the 400mm DO much earlier - but that is another story).
Speculation for me is, that future big whites will be RF-based and are likely to be even better on R-cameras than current EF lenses.
So whether to go with the already announced 100-500 mm or with some yet to come bigger whites can be open options for now.
Thanks again.
PS: Some of my Wild-life work can be seen at:
https://symphoto.smugmug.com/Wild-Life