I agree that taking in-person math classes at an actual four-year university sounds pretty good to me. Great experience for future in-person college classes, and it is entirely possible the profs will be quite good. And really, at most four-year colleges, the students taking that level of math are going to be pretty serious.
I also agree that MVC, Diffy Q, and LA are the natural next steps for a Physics major. I actually quite like this Ohio State chart which I think does a good job explaining what Math leads where:
MVC is Calc III, and people often do that next. Diffy Q and LA are then foundational, either alone or sometimes in combination, for a bunch more math relevant to Physics. If you are looking for more, Partial Differential Equations is a good one for Physics, or something like that foundational analysis class (3345 on this chart). But that other box down at the bottom also has stuff I would consider–like an advanced Probability class, or Vector Analysis. You will actually do stuff like that in college Physics classes for majors, but I think it would be great to have seen it before!