DNS does not rely on a order for its DNS servers. It simply queries the fastest one first and done.
Windows implementation, however, does slightly differ. It queries its first DNS server listed and then if it cannot resolve, goes to this alternative DNS options.
Could setting a order of DNS servers, for this purpose, be added to the GUI?
The DNS servers used by Pi-Hole as upstream servers are invisible to the client. Any settings on your clients are unique to the clients and not affected by the web GUI.
Client queries its assigned DNS server. If that is Pi-Hole, then Pi-Hole responds. If the requested domain is not blocked, then Pi-Hole (transparently to the client) goes to its assigned upstream servers for the answer. There is an algorithm built-in to Pi-Hole to determine which of the upstream DNS servers is the best performer, and Pi-Hole favors the best performer. This selection process is invisible to the requesting client.
If the Windows clients has multiple DNS options available, and one leads to Pi-Hole and the other does not, then if the Windows client chooses the one that does not lead to Pi-Hole, then Pi-Hole will never see that request and the client choice of DNS is invisible to Pi-Hole.
How do you envision your feature request working?