"Until about a year ago, we instead used a self-hosted PowerDNS on the back-end. PowerDNS was backed by a MySQL (later MariaDB) database. MySQL/MariaDB had been chosen for its support for streaming replication."

This is completely unnecessary (to say nothing of how MySQL silently corrupts data): PowerDNS can be configured for regular AXFR's between servers and that works exactly as one would expect, irrespective of what the back-end storage engine is.

The most effective, simplest, robust and therefore cheapest way to run PowerDNS is with the SQLite back-end as the storage engine and AXFR's between master and the slaves configured (PowerDNS BV run it with SQLite themselves); this inter-database replication is nonsense, just asking for trouble due to fragility and complexity. When I see such crazy unnecessarily complex solutions, I often wonder: what the hell went through those people's minds? Aren't they able to reason it out logically? What the hell were they thinking and why the hell were they thinking it??? This solution is so irrational, that it has to be ignorance.