Your dentist sounds understanding and really nice.
I think you were being very open and honest in explaining that you usually get unwell later in the year. I agree with Cinnabar_wing that it’s good to be realistic about future events and to prepare.
I'm glad your appointment went fine, and they were good with explaining and making sure you were okay. Having a nice dentist must make a big difference.
It's good to be realistic about the appointment, if it comes and you are unwell, it can be hard to book again for months, so getting it on the other side of winter is very forward thinking. You can't predict these things, but you can prepare as best you can!