https://github.com/doy/rbw Is an alternative Bitwarden cli front end. Probably has plenty of scaffolding to build a GUI frontend based on it.

Edit: Just a bit of googling turned up these as well.

https://github.com/AChep/keyguard-app https://github.com/sgolub/bitclient

There is not an alternative frontend that I'm aware of, but as the article mentions, the clients are Apache 2.0 licensed, so in the event of a rug pull, a fork and rebrand of the clients would be what is needed to restore service.

Better question is how difficult would it be to have keypass use vaultwarden for sync.

Their android app at least is open source and on available on their own f-droid repo