I’m using voyager, and I get prompted to sign in or sign up. I’ll have to try on pc later.

I have to login to Mastadon to see the kitty?

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