@Bill27 I cannot really help you with details. I did the Baztan in very nice autumnal weather.
If empathy helps, I can give you that
I finished walking Invierno a week ago and the rain throughout was unmerciful (but not snow) and two people walking behind me have had a continuation of the same. The words biblical, noachic and diluvial have been used!
And clearly at this time people are having difficulty or at least anxiety on Primitivo from what you can read on the subforums.
Rain is 'doable' it just isn't very nice, and obscures the views and on the Invierno converts some paths into rivers or mudbaths. And can make paths dangerous in parts, as I found out this year in Albania and in North Macedonia.
But obviously snow is a different thing altogether and it is important to stay safe and I suppose local advice can be very useful here. I have not walked in snow in Spain (and I note first snow of the year in both Roncesvalles and O Cebreiro earlier this week) but I did around Great St Bernard Pass in Switzerland last year.
There are two linked issues I think - the physical difficulty of walking in snow and the added difficulty of finding the correct path in the snow.
If I were there I would find someone local who you feel you can trust to give wise advice - I know this may not be easy. And I would err on the side of caution.
Can someone here tell us how good the direction marking is on Baztan if it is snow covered? I am guessing not as good as on Primitivo which had high posts marking the path over hospitales route which would stick up out of the snow.
Buen Camino and do stay safe and warm.