Hi,
Your app is really nice!!

I just miss a feature for easy sub checklists management without deactivating sorting.

My idea:
I use lists in the "note" section with "o" for unchecked and "x" for done at the beginning of each line. If you would implement a feature to handle these letters as sub-checklists, which would change "o" to unchecked tick box and "x" for checked tix box, this would be awesome. Activation/deactivation might done in the settings?

Pro:
x simple
x no issues with sync since simple letters are used in the background without graphic boxes.
x maybe easy to implement?