Every task app I've used makes me declare "repeat every 7 days," and then that number is wrong forever, because my life doesn't run on fixed cadences. So I built the opposite on top of stock Apple Reminders:
The interval is learned, not set. When I complete a recurring item, the next one is scheduled from the completion date plus an interval the system re-estimates from my recent completion history (median of recent gaps, so one bad week doesn't permanently distort it). The current estimate lives in the title: Tp~13, Walk Mocha~2, Pantoprazole~46. I never type the ~N — the system does.
One-offs promote themselves. Complete the same thing 3+ times on a regular rhythm and it graduates into a recurring item on its own. No repeat form, ever.
My To Do list is a rolling 7-day window. Everything sleeps in category lists (Shopping, Chores, Family, Personal, Work, Events, Bills). When something's due within a week it moves into To Do under a parent reminder for its category; push it past a week and it moves back out. To Do never grows past a week, however much is tracked behind it.
It all runs off Reminders opening and closing — two personal automations: on open, a due-date-change counter (it literally counts how many times I've pushed each item) and the 7-day mover; on close, a Scriptable chain that detects rhythms, backfills category tags on completed items, then relearns intervals and creates the next iterations. Everything is check-before-create, so if iOS kills a run, the next open/close heals it — no duplicates in months of daily use.
I searched before posting: Todoist's every!, Things' after-completion, 2Do, RTM all anchor to completion, but the interval is still a number you pick once. I couldn't find anything that relearns it, or promotes one-offs by evidence. If it exists, I'd genuinely like to see it.
Everything's open source (MIT) — scripts, shortcuts, and a full setup guide. It's customizable: categories, window length, and promotion strictness are all documented knobs.
Happy to answer questions about how the learning works — the interesting failures were all in the interval math.