Things were getting really slow with so much upstream DNS rate limiting, that I just took the plunge and migrated my Pi-Hole configuration from OpenDNS to Quad9. It's not perfect, but it's better (for now, until they decide to rate limit as well). Still need to do further investigation to understand what I am losing or gaining with that change, in what regards to filtering.
Bonus points: I never realized that Pi-Hole doesn't know that home.arpa is officially considered a local DNS zone, so it was forwarding queries to upstream as well. This is now blocked, so suddenly everything got a lot faster.