You say Celery can use Redis or RabbitMQ as a backend, but I've also used it with Postgres as a broker successfully, although on a smaller scale (just a single DB node). It's undocumented, so definitely won't recommend anybody using this in production now, but seems to still work fine. [1]
How does Hatchet compare to this setup? Also, have you considered making a plugin backend for Celery, so that old systems can be ported more easily?