Hmm. The Encryption Password for the Web Edition is merely to encrypt the configuration that holds the credentials for your storage - it’s not to protect the files you’re wanting to backup.
You’re right that with complete access to your PC, an attacker could potentially decrypt the keyring and ultimately get access to those storage credentials.
However, that’s with complete access (not always the case) and they’d need to find and reverse engineer the configuration to get the raw credentials, by which time hopefully you’ve bought yourself a little time to revoke B2 tokens etc… (Plus, you can opt not to store that password in the keyring, then an attacker would need to install a keylogger before you input it each use.)
A few things have to go right for an attacker to gain access to your backup storage in good time to do anything with it, so while they may have access to your files, it’s important they eventually don’t get access to your backups so they can hold it to ransom by re-encrypting it (or simply changing the master password)!
Furthermore, this password isn’t crucial to gain access to your backup. If you forget it, no biggie. Just reinstall / delete configuration and setup the storage again. The only password you need to keep safe is the storage password (and ofc access to your B2 account). Thus if you’re not particularly fussed, just choose a simple Encryption Password for the Web UI - like “none” - effectively the same as not having one.