1) has multi-platform, multi-device support, and
2) has the ability to store encrypted password vaults in user-defined 3rd-party cloud file storage like Backblaze, Cloudflare, Google Cloud Storage, AWS S3 etc. much like how Arq Backup and other backup software allow user-defined 3rd-party cloud storage.
Got any suggestions or experience with using any such password managers?
Or bitwarden plus the self-hosted server bit.
If the local directory it uses is mapped (via appropriate means) to one of the above cloud providers (i.e., a "dropbox" style mapping) then the local encrypted file is also stored "in the cloud".
Note also you could simply setup "backup into cloud" of the local file and achieve a similar result.
Here you will find a long list of compatible other variants: https://www.pwsafe.org/relatedprojects.shtml some of which are for multi-platform and/or multi-device support.
I'm not only thinking of myself but also my family. KeePass is not user friendly the way 1Pass is and that's very important.