If you own the phone, the messages are decrypted. This seems to be more about the phone access than anything to do with Signal, right?
Seems like this is partially Signal's fault to me. Why doesn't Signal independently encrypt the message db when not in use? It's well known at this point that iPhone is easily cracked, and thus Signal provides no security for a stolen phone. Edit: it seems that Signal uses db protection but in a way that fails for a cracked phone like this (?): https://news.ycombinator.com/item?id=26096778
To what end?
If there's no additional authentication required to open the app and just view the messages normally, it's useless because they can just open the app and view the messages.
If they use their existing PINs as key material then, given an OS-level exploit grants access to the underlying database, it can be cracked offline in... basically no time at all.
If they allow you to create a long, secure password for opening the app, basically nobody uses it because nobody wants to type a 30 character password on a phone touchscreen every time they want to check/send a message.
Pretty much any solution that doesn't absolutely destroy usability still relies on the OS performing some sort of authentication first. (As in the current solution where the encryption keys are stored in the OS-level key store.)
If you're that concerned about your security, just go disable FaceID/TouchID so the iPhone never leaves keys in memory when it's locked.