autoremind.ai vs Boomerang
Boomerang is the oldest tool here and it shows. Years of polish inside Gmail, a genuinely free Basic tier, separate apps for Outlook, iOS and Android, and an install base large enough that most people asking about follow-up reminders have already tried it. Send later, snoozing, recurring messages and returning a thread to your inbox if nobody replies all work as expected, at $4.98 a month on annual billing.
If you want a dependable scheduling and snoozing layer on a mailbox you already like, Boomerang is the better buy and we are not going to pretend otherwise. It is also the only tool here with a real mobile story.
The difference is what happens once the reminder fires. Boomerang hands the thread back so that you can write the next message. autoremind.ai writes it, sends it from your own Gmail into the same thread, and makes each message firmer than the last until somebody replies. One returns the task to your inbox. The other finishes it.