I was able to get a digitally sovereign, self-hosted contact form going for my Ghost site, by installing GitHub - styxlab/ghost-contact-form: Contact Forms in Ghost — Without External Services · GitHub in a docker container on the same server. It has its own subdomain and uses a dedicated account on my mailcow SMTP server to send the message.
The code is easy even for a non-programmer like me to follow, and I was able to change it so the email is delivered straight to me as though it had been sent from the sender’s mailbox. This is going to be great for responding and having a conversation!