Hello @pierre.tillement,
Complementing @johnny great suggestion, please review the following:
It seems that nested repeating groups could do the trick.
First RG: tasks
Nested RG inside each row of the above rg tasks: lists the things that you check a checkmark for
Just a guide to spark ideas, please consider something like this:
Database
Task thing with a title type text and a field with a list of subtasks
Subtask thing with a title type text
UI
RG tasks with a nested RG of subtasks:
Hope this helps! 