What is it like to drive between Frankfurt and Strasbourg around Dec. 18 to 26, 2019?. I want to drive leisurely and stop at different Xmas markets. Xmas markets close on Dec. 22. Is this correct?
Are there cheap hotels to stay along the way?

It will be cold.
I can't comment on hotels.

The Market in Frankfurt is great. In fact, Frankfurt is great; don't short change it.

Favorite of several Christmas markets near Frankfurt---We really like the one in Michelstadt, with more hand crafts and a village-y feel than Frankfurt. It's not on a direct route but is south/ a bit southeast of Frankfurt; I'd say try to fit that in.

I don't know when specific markets close but need to confirm the dates of them so you are not disappointed if want to attend or not bothered by crowd if trying to avoid.

If you have time, I highly recommend Mainz, Worms, and "of course" Heidelberg in between Frankfurt and Strasbourg.

We did an Xmas markets trip across Germany a few years ago (Hamburg - Wismar - Schwerin - Hildesheim - Hameln - Münster - Düsseldorf - Aachen, then Belgium and the Netherlands). At the time it wasn't that cold and we had no snow, so we just had dumb luck. However, I do want to put for you: how experienced are you with winter driving conditions, as at that time of the year there could well be snow and icy conditions, and driving might not be a great way to see the country. In other words, you might need a plan B. But our trip was pretty good and we noticed subtle differences in markets as we crossed the country.

Of the markets between Frankfurt and Strasbourg I have only been to Mainz, but it's a pretty good one.

Lavandula

The Strasbourg Christmas market runs until December 24th.