I've camped & toured in NL in the winter and loved it! But I lived there and they rarely lasted more than a long weekend.
There are campsites open (not many so distance can be a challenge) with some excellent facilities such as kitchens.
€20 per day is possible, most of that taken up with camping. I'd question why travel to such an expensive country with such a low budget at a time of year when there will be minimum human contact unless you can eat out or at least buy a coffee?
Weather can be very variable. I've cycled in -10 C temperatures at that time of year. Snow could be a big problem. Rain and a lack of daylight are always issues. Wildcamping is a no-no, especially in a post Covid world.
(By the way, restrictions aren't finished)
Working lights will be essential or expect a fine.
The end of December will be very, very busy ( New Year celebrations) and expensive and will likely impact any warmshowertype options. January, on the other hand will be very, very quiet.
For a first tour I don't understand why you'd choose NL at that time of the year. The south of Spain would be far more tour friendly. And warmer.
Or head south to México!
And Covid hasn't gone away.