Second, a nice way to decouple the 3 steps you said is with "functions, serverless", it fits your example very well ( async events that triggers 2 separate functions).
First point again, a few notes:
a new dependency is not such a big deal, you will gain benefit from it from multiple systems (I bet this 3rd party separation is only the first for more to come)
relational DB aren't such a good fit for this communication type
you will put a bigger load on a persistent DB with transient messages
you cannot scale it horizontally (except by doing separate databases for each "channel")
from what I know "LISTEN / NOTIFY" sends the message to ALL listeners, this means you will have the problem of "at least 1", you will need a PUB/SUB system that does "almost exactly 1" so you will send only 1 email. If you only have 1 nodeJS server that listens again you cannot scale.
by not using a proper messaging systems you guys need to develop a failure/retry and timeout system (if one worker failed it must be reintroduced in the queue as a "not done job")
low performance - most messaging systems are in RAM so you can't match that
if you rely on the trigger/instant call of the queue update you will actually build a (time) tightly coupled system, systems like RabbitMQ/Kafta allows a "decoupled system" in time, allowing a greater flexibility in the workers usage (they can all be down and resume later or handle spikes in time).
PS: I may be wrong, I'm not deeply familiar with PostgreSQL
PS2: there is no such thing as zero dependency, if you guys do this system in house you are creating a new project (in project), that requires your attention, tests, time and resources. Also the system is dependent on your business logic, infrastructure and database (things that can be mitigated by using a PaaS / cloud PUB/SUB))
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Wow, first of all don't do it.
Second, a nice way to decouple the 3 steps you said is with "functions, serverless", it fits your example very well ( async events that triggers 2 separate functions).
First point again, a few notes:
PS: I may be wrong, I'm not deeply familiar with PostgreSQL
PS2: there is no such thing as zero dependency, if you guys do this system in house you are creating a new project (in project), that requires your attention, tests, time and resources. Also the system is dependent on your business logic, infrastructure and database (things that can be mitigated by using a PaaS / cloud PUB/SUB))