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What also could work for you is http://todotxt.org/, its basically a text file or two if you want to keep track of some tasks, it already comes with two types of labeling but also key value tags, which you can use to filter based on your different desired levels.
https://tasks.org/ has today and atleast 3 levels with lists, tasks, subtasks. the fourth one you could do yourself via title prefix or tags.
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.