Sort of sounds like you’ve answered your question here. 21 recommended and 24 too wide, so 21-23 depending what you find. Probably preference for the 21 end of that spectrum, but I say up to 23 for two reasons - one is the Challenge website says all their tyres are now made to remain UCI compliant on up to a 23mm rim - you’ve said that doesn’t really matter to you but it’s an indication of their design thinking. Also, rims are getting wider rather than narrower, which means the next few generations of tyres are going to be optimised for wider rims too. So 21 might give you the best ride today and 23 might start to approach that square feeling, but 23 is probably also future proofed in terms of tyres for a while.
(As for how wide CX rims will go, my theory is they’ll keep going til they hit the 105% rule - even if CX is relatively slow, there’s still enough time spent on course at decent speeds for there to be some kind of aero savings by having a 33mm WAM tyre on a 35mm external rim which is 45mm-ish deep. Hard to say whether that will be a 30mm-ish internal rim width or if they’ll keep it in the 25mm range and then just mould extra carbon onto the rim like the current Roval Rapide wheels.)
There’s some confidence to be gained by having hooked rims if you have the option to go either way, but hookless interfaces are getting better and better so if your ideal wheel happens to only come in hookless I wouldn’t let that be a reason not to buy it. Tyre inserts could be a nice extra security feature if you buy hookless and end up having issues or feel unsure at any point.
In my experience, there’s no measurable difference in tyre width between otherwise-identical hooked and hookless rims - the difference is really just in the shape of the inflated tyre, and that’s almost too small to notice anyway.