I'd just use heroku and pay a bit more(or nothing if no users?), use plugins for what you've mentioned etc.

The cost of running is marginal and if I'm lucky enough to get a lot of users, I can easily switch to anything.

There is a lot of dev cost to wire up everything you've mentioned. Serverless is very devtime-wise consuming, it's state of art tooling is still slow as fuck if you compare writing an app using Rails/Django/etc.

IMHO dev time is what hurts more running a startup, this is what I would try to reduce