Sounds like you need to write an app to do this. It seems you want to aggregate data from multiple sources, keep secret, then send.

I’m tech inclined, but not a developer, lots of low/no code solutions should let you get pretty close fairly easily.

I’ll bet you could use a jobs site to “rent a developer” and it wouldn’t be too expensive.

Or find a 3rd party to collect and schedule the email.

I think my email service does something like this, though I don’t use it.

https://proton.me/support/schedule-email-send