You mean the password manager as the central authority? You can self host a password manager using, eg, Vaultwarden.
Even if you use a trusted, paid commercial service, I think the risk of that happening is lower than on Reddit. Their business model is simpler and more transparent. They want to keep you as a customer so you will keep paying them. And there is less opportunity for them to ban you for political reasons when you’re not expressing yourself on their platform.
While I mostly agree with this, I would point out that mandatory TLS introduces a decent bit of complexity, both in implementing TLS itself (where you should really use one of the established TLS libraries in your language of choice) and in figuring out what to do with certificates (TOFU, etc).
It’s still a very simple protocol of course, but not quite so simple that you can negotiate a connecting manually over telnet, for example. (Some versions of netcat, on the other hand, do support TLS.)
Services vary a lot on how they are deployed and their dependencies, etc. The knowledge I have (and honestly I don’t have much) I just built over time, tinkering with different set-ups and trying to debug problems when they arose. So I guess just choose a few difference services and try to get them working (choose low-stakes ones at first, where the risk of getting pwned or losing everything is very low). Docker can abstract away a lot, so maybe try more direct deployments if you are interested in learning.