If you don't want to completely abandon Google's ecosystem, use Tasks.org as the companion app. Since it syncs with Google Tasks, you can keep using Keep for your regular notes and Google Tasks for ...

also something for 'sequenced, associated tasks'? calendar / reminder software where if you move task #1 from 10am to 9am, then task #2 that is associated with task #1 will move from 11am to 10am ...