Congratulations!
Acknowledging the pronouns used in the situation, I can get confused when we're talking about a program or a school (which T5 schools are being referenced).
I ask because I would like to know what the partner's medical school's policy is when it comes to transferring a student out of the program. What has their student deans said?
Chances are that both student support offices are familiar with each other, and they can hold a confidential conversation about whether there is a path to having you both together when it comes to starting a family or completing the proper phase of study.
One option is to give you the deferral to the 2026 class. Your partner would have completed M2, and, I hope, have passed Step 1. If so, perhaps there are ways to do out-rotations at your school's affiliated hospital. It sounds easier than it is, and that's where those private conversations between the deans (and others) are important.
There will also be a question to approach with both financial aid offices. I presume that the partner's financial aid package would require a significant adjustment.
I would also check both student handbooks about family leave policies. Would the partner take a leave to be with you, and how long? As I pointed above, S1 is a significant gateway and inflection point in medical education, so how does this affect their timing to get to S1?