Bitwarden is open source (server, plugin and app) and can be self-hosted so it's not centralised in any way that matters.
Also, I think an honest freemium offering is the best way to do it - have those that are willing/able to pay subsidise those who aren't. It doesn't have to be a slippery slope, and that's not exactly common in the open-source world. After all, you can just fork it and go your own way if you're not happy. Also, running servers isn't free, and being able to remunerate the devs a little is no small thing.
So, in summary, use Bitwarden. You can set up your own server and install the plugin/app yourself if you want.