If you're a 'startup', you'll never need any of that work until you make it big. 99% of startups do not make it even medium size.

If you're a small business, you almost never need replicas or pooling. Postgres is insanely capable on modern hardware, and is probably the fastest part of your application if your application is written in a slower dynamic language like Python.

I once worked with a company that scaled up to 30M revenue annually, and never once needed more than a single dedicated server for postgres.

I don't think any of these would take more than a week to setup. Assuming you create a nice runbook with every step it would not be horrible to maintain as well. Barman for backups and unless you need multi-master you can use the builtin publication and subscription. Though with scale things can complicated really fast but most of the time you won't that much traffic to have something complicated.

Why would your distro dictate the upgrade routine? Unless the distro stops supporting an older version of Postgres, you can continue using it. Most companies I know of wouldn't dare do an upgrade of an existing production database for at least 5 years, and when it does happen... downtime is acceptable.