All the campsites seem to have 2 day minimums and most seem geared to caravans, not travelling bikes with a small tent.

What's your source for this information? In my experience NL is very familiar with the idea of cycle camping. 

Also there are some camping sites with just a small green tent instead of a green tent and a marker blob. Does this mean anything?

Is this using the 'find accommodation' feature on CT? Tent symbols on the map can be many things tangentially related to a campsite. Tent symbols suggested by CT  using the 'find accommodation' feature will be what the OSMand database is showing, Other resources are available - Google for 1. In NL I'd be surprised if any but the largest, resort-like sites have any issue with bikes. 

As for 2 night minimum that's new to me. Have you tried speaking directly to the sites? You haven't said when nor how many people. The when might be important for that. If only 1 or 2 cyclists I wouldn't bother making reservations - they are normally very obliging. 

Hi I'm not sure thus is still relevant for you but I’m Dutch and cycle and camp a lot in the Netherlands. 

Some campsites have a minimum stay but this is usually only in peak summer. And its rare.  Also in my experience walking and cycling travelers are almost never refused when arriving without reservations as I myself do. I’ve never been refused and I’ve camped here a lot. 

Consider making use of natuurkampeer terreinen. These are small campsites which are located in natural environments and often geared towards tents. You need a membership -15,- or so- which can be purchased at the campsites. You only need this once for all campsites. Camping itself is cheap. 
https://natuurkampeerterreinen.nl/?lang=en

ciao Joost

Useful information Joost, bedankt