Nonetheless, we just spun up a Route53 zone, exported our zone from DNSimple, imported to Route53, and hand-migrated our ALIAS records to static A records in the new zone.
Not perfect or permanent, but we've gotten around the outage. Also, I just learned that pointhq has (seemingly-undocumented) support for ALIAS records in the same style as DNSimple, so this could be another avenue to explore.