RF 400, or EF 400 with adapter?

Well, you know more about RF than I do, and I have been shooting Canon for 35 years.

First I am shocked they are the same price.  Canon usually prefers to gouge it's customers.

It is a large purchase to consider...not just the size of the lens.  I have used mirrorless in the past so my main concern is how well you can adapt to how they work for sports.  I will make that assumption based on it being the 400/2.8, and I am referring to the EVF.  I would master the R5 first to evaluate how it is likely to perform with a big lens.  This will also tell you how much you rely on the control ring.  If you end up preferring an OVF then go with the EF and control ring adapter, and buy a used DSLR for those times when you prefer the OVF.

If you evaluate the R5 and determine that you are never going back to  DSLR then I would chose the RF lens, with one caveat.   It certainly looks like the RF 400/2.8 is not truly an RF lens.  It seems to be an "affordable" stop gap until Canon themselves get used to the RF system and what they can do with it.  I suspect that with continued success of the RF system that Canon will replace this lens before too long.  Perhaps with even a 400/2.4.

If you chose the EF 400/2.8 I suggest purchasing a used IS II for half the price, see how it goes, and then resell it at almost no loss, when you see what Canon does with an updated RF 400.

I have used two old 400/2.8 lenses so I know they are all incredible!  Any advantage that the current RF lens has over the EF will be virtually unnoticeable.  It is the replacement for this RF 400/2.8 that could be the deal breaker.