Are we crazy to go to Strasbourg and Colmar in December?

The Christmas Markets will be crowded, but other sights not necessarily so. It is not crazy to go there at that time, in fact it is arguably one of the best times to go - the markets are fun.

This being said, if you want to add French destinations to London + Paris and avoid crowds, you could head to Provence. Often sunny (and windy, beware), and definitely uncrowded at that time of the year.

We went to these markets in 2019, and it was one of the most magical experiences in my life, and I'm 70-ish. I really don't remember the crowds being unreasonable.
The crowds are worse as the locals get off work in the evenings, especially around the gluhwein stands. Weekends might be more crowded than weekdays.
Mornings and afternoons were not crowded at all during our visit. In Strasbourg, there are probably a dozen markets, all spread out around a walkable area. Shop during the day, and view the buildings, which decorate each floor, up to the roof. Then in the evenings, wander the pedestrian alleys, arranged in a spoke- like fashion around the Cathedral. Each have waves and waves of lights, each alley is different.
We stayed in the old town, and it was nice to easily return to the room to warm up and put our feet up. I would advise you to book hotels asap, as I found some places fully booked by June.
Have a wonderful time!

We also spent a day on a wine tasting tour, which was great fun!

https://community.ricksteves.com/travel-forum/trip-reports/christmas-markets-switzerland-france-germany-2019

Thank you all! I’ll keep our itinerary intact. I really appreciate the feedback.

All hotels already booked and staying right in the heart of things. We’re also mid week.

Oh, if you are going mid-week then you do not need to worry that much about crowds I guess. I never had the chance to go mid week!

I lived in Germany for four years and did not like the long cold Winters.
It gets dark at 4:30pm and you have to wear gloves, overcoat and warm hat.

Perhaps, if you visit museums where it is warm.

We’re from Oregon where it’s dark before 5:00 in winter and lots of rain, so we should be fine. Plenty to do indoors.