No, it wouldn’t be mandatory to move. You might be advised to as we were. I know an SSPX family who were Third Order SSPX (these people are actually members of the SSPX as laypeople, while most of the rest of the parishoners are not) who moved more than once to build missions - under obedience. But I think they were still allowed to consider the prospect, but with more pressure due to obedience. Ultimately, we all have free will. Personally, we were convinced that, for our children’s spiritual welfare, we should follow this advice from the priest.
I don’t mean to come across as emotional. I’m actually at an emotional distance at this point. I also have nothing against home schooling (we did for many years), modesty, and the TLM. The message I was trying to impart with those topics is that those things are a necessary and mandatory part of the SSPX culture. For instance, if I sent a daughter out to play in the muck with pants on, even though she usually wore a skirt, I would be criticized. When I cut a daughters hair to shoulder length, I was criticized (it wasn’t considered modest). Or when our family had a situation which made it necessary to send children to diocesan Catholic school, we were criticized. We were also advised to, when there was no SSPX Mass on a Sunday (mission territory) to stay home or pray a Holy Hour in the chapel, instead of fulfilling our obligation at the Parish Church.
I say these things to identify the culture.
I would also like to mention this. When we say that the SSPX is not “outside” the Church, there needs to be a clarification. We had a number of our children baptized and confirmed in the SSPX. When our family returned to the Diocesan Church we were required to have our children formally registered as Catholics, and to make a profession of faith publicly. They had to enter like a Protestant or an Eastern Orthodox convert. No, there was no question of “re-baptism” or “conditional confirmation” - those sacraments are considered valid. But make note that they were not considered in union with the Catholic Church. They were not considered to be technically Catholic. And none of my good intentions changed this fact. Imagine my dismay…
So, while a marriage could be convalidated later, I think a couple who *understood *the situation would be committing a mortal sin to go ahead with the sacrament in this way.