I've been frustrated as well with that; my mind is inherently hierarchical/nested, so when I plan, document or checklist something, I neeed levels and indentations.

I've been using ToDoist for a while now and it's worked well. Some of the recent features/changes have clearly been for people with workflow different than mine, but the overall easy-nasting of tasks/subtasks to multiple levels and easy-drag-drop rearranging works well, and it's available on both web and phones.

todoist.com/app/