Recommended Shake256 digest size for KMAC based HKDF
HKDF already has expansion on top of the MAC that it is based on. What would be the point of using a XOF for that? Why not use the XOF directly or define a different KDF? Because having two different methods for expanding the output seems troublesome to me.

If you're going to use HKDF then use one of the normal SHA-3 / HMAC functions with it. Sure, that's less than optimal, but commonly that's not a huge issue for deriving a limited amount of key material anyway.

Any form of HKDF that redefines the PRF used to derive the key material will not be compatible with anything out there. That's true regardless of the XOF and KMAC standardization.