Bitwarden

The proton pass client is open source, not sure about the server. Keepass, in it's many flavors and permutations, is always an option, depending on your definition of easy. Firefox Sync, too, not sure what happened with lockbox.

I use the open source bitwarden server vaultwarden, but still use the official browser plugin and Android app - though I wonder if this library license change would affect open source client implementations by requiring third party extensions and apps to implement their own compatible BitWarden API. The cynic in me wonders if that could be a move to make it harder on developers of third party clients in an effort to strengthen the position of the official clients even when using a third party server like vaultwarden. I never really considered using a foss bit/vaultwarden browser extension or Android app but maybe I should if it exists.

Someone correct me if I'm interpreting the license issue wrong.

Proton is safer from enshittification imo, it's not as open source (backend is closed), but it's their lack of VC funding and the majority owner or the company being the Proton Foundation non-profit that reassures me. They also have an history of lowering their prices if they come down, like with Pass. Other than that I think 1password is also very good in terms of security, though not open source

The fact is, both are still probably less open source compared to BW, but at least they don't use open source as a marketing point