System design hack: Postgres is a great pub/sub and job server
I did something very similar with MySQL since that was the DB we already had setup for data (not my choice). Basically any database that has atomic operations can do this. It definitely is way more convenient and cheaper to do use your existing infra for pub/sub and then only scale out to other services once performance becomes subpar. Although if you already have a messaging service up and running, it’s probably better to use that