Share to/from WhatsApp has a feature where you can send a message from WhatsApp/email/Telegram/Facebook messenger and automatically make a note. It accepts photos too.
Unfortunately, the data can’t be exported yet, so that site appears to have no use.

Still, there should be a way to set this up for Obsidian, possibly using existing tech of some sort. All we need is a bot that can read messages and save them locally into the vault.

you are saying that has a dedicated WhatsApp phone number that you can write to?