At this point, there is no real reason not to let Ukraine in, given how closely tied to NATO it has become and that, as Zach says, Russia already considers this a war with NATO.
Maybe it's time to show Russia what a real war with NATO looks like, and show we aren't scared of Putin's bluster anymore.
If Putin breaks the nuclear taboo for a tac nuke to try to 'escalate to de-escalate', it'll just alienate Russia even more from it's few remaining friends and remove any lingering divisions in the groups opposed to Putin's madness.
Russia respects only force, and Zelensky has made clear there will be no further negotiations with Putin.