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.