full bridge llc
I understand, that you wonder, if the said 420V/2kW/250kHz are a profound design, or just a concept, without considering details as switching losses. I also don't know in this case, cause the originator is only asking for the transformer design without mentioning any details.
But I'm sure, that it can be designed, as a hard switcher and as a resonant converter as well. All these reservations in your post, that you can't do this or that, are basically application specific design parameters, without general validity. You can find contradicting design examples for each of it.
It can be meaningful to opt for 100 kHz switching frequency rather than 250 after calculating the losses for an existing power stage, of course.
Regarding the original question, it's rather easy to find a suitable core, either for 250 or 100 kHz. I think, the winding design is actually more critical, and it' necessary to know the available manufacturing options, e.g. flat wire, foil, litz wire and also to specify winding isolation, possibly shielding, acceptable leak inductance.