Well, I’m not sure you read the other comments but there is confirmation that for clients there isn’t an order for DNS servers from RFC2182:
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.
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.