Re-applying to Math PhD programs while in first semester of current program. How viable?
There is really no answer to the &quot;viability&quot; question. The only thing to do is to apply and make your best effort to be accepted.
But, as you probably already know, getting accepted to a single institution, considering no others, has a pretty low probability of success generally, due to the competition.
But there is really no reason that you should't apply. If you are turned down you are no worse off than now. Most people will, I hope, understand your reasons.
But there may be another path open, if your partner is in a place with several universities in reasonable distance. You might apply to several universities within, say a 50 mile radius. In some places that is a pretty large set of universities. Not everywhere, obviously.
Your partner can take similar action. Maybe you'll get lucky.