I would do it manually if I could, really. But it is not humanly possible without stopping work for a couple of days on the repo.
I think you don't have bad intentions. A "thank you" email looks like a nice detail, but I would consider it borderline spam. I'd probably ignore the first one, but if I ever get a second one I'd hit the "mark as spam" button in gmail.
I wasn't sure of the correct social etiquette there. Yep I made it clear that it was a one-time thank you email, and do think that it borderlines as spam.
The receiver hasn't given you the ok to e-mail them, and sending mass unsolicited e-mail is basically the definition of spam. If you want a special way to contact users, have them clearly opt-in to it. Could be an e-mail list, could be a github issue that you ask them to subscribe to, ...