Open UDP ports are pretty secure and rarely found by scanners. The basic issue with scanning for UDP is, that most services don't respond to random garbage you try to probe then with. Without getting a response back, the scanner has no way of knowing if there is something running on that port or not.
Wireguard in particular only responds if the correct key is given.
Also make sure your firewall DROPs (usually the default, but do check) disallowed connections instead of REJECT. This way any UDP probing, whether it's to an open port or closed one just times out with no way for the scanner to distinguish them.