Agreed with using keepass. If you’re one person accessing your passwords, there’s no reason you need a service running all the time to access your password db. It’s just an encrypted file that needs to be synced across devices.
However, if you make frequent use of secure password sharing features of lastpass/bitwarden/etc, then that’s another story. Trying to orchestrate that using separate files would be a headache. Use a service (even if self-hosted).
Let’s Encrypt is good practice, but IMO if you’re just serving the same static webpage to all users, it doesn’t really matter.
Given that it’s a tiny raspi, I’d recommend reducing the overhead that WordPress brings and just statically serve a directory with your site. Whether that means using wp static site options, or moving away from wp entirely is up to you.
The worst case scenario would be someone finding a vulnerability in the services that are publicly exposed (Apache), getting persistence on the device, and using that to pivot to other devices on your network. If possible, you may consider putting it in a routing DMZ. Make sure that the pi can only see the internet and whatever device you plan to maintain it with. That way even if someone somehow owns it completely, they won’t be able to find any other devices to hack.