The reminder is the easy part. The hard parts are deciding your allocation and selling winners to buy losers. No app does those for you.

- Two or three funds? You probably do not need one; drift moves slowly - Five or six funds across accounts? The reminder earns its keep - Set drift triggers ("tell me when stocks exceed 85%"), not calendar nags - A nagging daily reminder is a bug, not a feature; quarterly checks beat constant fiddling - Rebalancing has a tax cost in taxable accounts; calm wins

Overlap ($4.99 one-time) helps with the part before all that: the best ETF per slot, fees and overlap side by side.