Note that you could have a UK State aid regime based on EU law (preserving continuity and minimising the huge/unnecessary uncertainty that follows from introducing a brand new set of concepts trying to do much the same thing) while not being EU law (so not requiring an ECJ role).
Think of the way in which Australia etc took over English law on independence and then developed it in its own way.
That would also make it easier to manage the problem of applying two potentially different regimes at once (given that the UK remains to a large extent bound into EU State aid rules for ever, given the NI protocol).
Indeed, that is precisely the plan (as I understand it) for competition law - keep the current prohibitions on abuse of dominance/anti-compete each agreements based on EU law and adapt as we decide to over time.
