Cancel anytime — it stops the next renewal, and your service runs to the end of what you've paid for. No retention maze; reply to any receipt or write to hello@onesmallprompt.com and it's done.
Going quiet is also fine. If you stop visiting, nothing destructive happens: the site, address, files, and your engineer's memory of the project stay intact. The engineer does no background work (and spends none of your allowance) while you're away. Come back in a month or six — it picks up where you left off.
Before you go, know that your whole project exports in one click — we'd rather you leave with everything than feel kept.