When it comes to a “secondary”DNS... there is nothing like a primary and secondary DNS server. These indications are quite misleading but many systems adopted it this way. Pihole only list the DNS servers as primary and secondary, because this is what the providers write on their pages. The bad phrasing is supported especially by how Windows handles it.

Most operating systems implement DNS servers as alternatives, not as fallbacks, i.e. they will query any of both servers from time to time, so it is quite likely that you will loose your Pi-hole filtering capabilities (at least partially) [if you specify a secondary DNS server on your network].

The ONLY DNS server you should have set on your network is a/the PiHole(s).

A number of problems in DNS operations today are attributable to poor choices of secondary servers for DNS zones

That RFC is about DNS servers in the context of DNS zones in domain names. Not for a DNS client running DNS queries like an operating system. And the RFC is very clear about this:

The distinction between primary and secondary servers is relevant only to the servers for the zone concerned, to the rest of the DNS there are simply multiple servers.

For your Windows, Linux whatever machine "there are simply multiple DNS servers".

Which defines/influences how resolvers behave with multiple DNS servers.

It doesn't. As I said in another comment "most operating systems will just load balance or opportunistically pick between the two", and there's the relevant part from the RFC:

All are treated equally at first instance, even by the parent server that delegates the zone. Resolvers often measure the performance of the various servers, choose the "best", for some definition of best, and prefer that one for most queries.