The concept of sub-tasks makes sense as it breaks down bigger tasks into small. I actually made my own app based on goals oriented habits, tasks and sub-tasks until I pivoted. I also added async social accountability by sharing snaps of whatever you're doing. I still have the code in the current version but I'm no longer targeting that audience anymore. It's part of something bigger. Shameless plug -> https://atomiclife.app. Still working on version 2 (technically version 4 as I pivoted 2x) and I build in public, you can follow me on twitter: https://twitter.com/rish1_2