Personally, having my sensitive data under my own control (but internet facing) terrifies me. I know enough to know that there are risks, and yet wouldn't have a clue about how to make it secure.
My self-hosted bitwarden server is only accessible from the LAN. Since the full password database is cached locally on each client, you can use it to lookup existing passwords just fine without a connection to the server. Bitwarden does require a connection to the server to add passwords, as it isn't a distributed architecture, so this setup does prevent you from adding new passwords while you are out and about, but I don't have the need to do that often, and in the rare occasions when I do, I write them on scrap paper in my wallet till I get home.
For home use, and to a limited extent - when all your users are proficient - for corporate use, I really enjoy pass (https://www.passwordstore.org/). It has a decentralized architecture where passwords are synchronized via git, making it excel at situations where you need to generate or store secrets on the go. Unfortunately, the Windows client is not stellar, and the (unofficial?) Android app doesn't seem to have an option to encrypt secrets using more than one key, limiting its use for most teams.
I got really excited about pass for a bit and almost switched to it, until I realized I was likely increasing my attack surface because in addition to trusting the developer of pass, I also had to trust the developer of whatever other third-party clients I was using with it (such as the iOS client).
My concern about this is less about making it secure but keeping it secure. Zero-day vulnerabilities are a thing and you can never be 100% safe against those, so the next best thing is to have good monitoring in place so you get alerted when something nefarious is going on. This unfortunately requires 24/7 monitoring that's better left off to a dedicated team.
If you're going to host services as home such as your password manager, set up a WireGuard VPN, you can use a Pi and it'll be perfectly sufficient, leave only the VPN open on the internet, VPN in from your phone, laptop, whatever for anything you need access to, and you don't need to rely on Nextcloud or Bitwarden having vulnerabilities discovered in them.
I was using Nextcloud previously for password sync because my password manager needs WebDAV, it was too much to maintain so I wrote a small server in Golang using the WebDAV library and it sits behind NGINX which handles the auth. I run Minio (S3 compatible) for syncing our family photos from our phones and Folder Sync app on Android. They both run on a VM and write out to a ZFS pool.
I have a Pi 3B+ running Raspbian mounted read-only as a WireGuard VPN for remote access, and we use the official WireGuard app. VPN is always on because we have fast, symmetric fibre, and we don't need to worry about trusting public networks.
Why would you mount read only, out of interest? How do you keep packages up to date? And what about logging? I'd want to be logging connection attempts.
To save the SD card mostly. I log to a ring buffer in RAM.
I've had my Raspberry Pis kill dozens of SD cards over the years, so read only can helas for updates, I manually remount read/write when I do maintenance and then remount read/only again when I'm done.
Thanks. I might have a look into doing it for mine, since that seems to make sense. I guess like you said you can just mount the fs rw, and then chroot in to run updates.
No need for chroot, I just say in when I'm on the LAN, remount / and /boot r/w, run the updates and reboot (boots any new kernel and switches back to r/o).
I would set it up locally on raspberrypi with PiVPN and only allow specific IP (bitwarden IP) to be accessible via that VPN connection. Also you will need self-signed cert installed on the devices where you want to access bitwarden if you dont have a public domain.
