Hi, your query should find this route that I know: https://www.openstreetmap.org/relation/6387227
Mind you, they’re relations, not ways.
And you’ll have to use some filters on network (ncn) and name/ref (F*) because else you’ll get all of the recreational routes as well.
Including the network=ncn in your query might help?
There aren’t many snelfietsroutes around to start with, and it seems you found many of them already.
Some may not use the ref tag, only the name tag.
Others may have tags that I don’t know of.
Ah yes, you found the recreational national routes (ref/name=LF*) now as well. Better leave those out.
I’ve looked around a bit in the Benelux, and F plus a number seems to be the naming standard. In the Netherlands, it’s often the number of a highway that runs parallel (A59 → F59 in my example), or a variation of that. Belgium seems to use the same. There are a lot more F routes in Belgium by the way!