I don't know how useful it will be, but i like superproductivity. Its foss, its flexible, one task can be assigned to multiple tags. There is a bit of a learning curve, especially with the android app, but for me its absolutely worth it
Unfortunately I have tried FOSS for this too, but have only been able to get all my boxes ticked (pardon the expression) using TickTick - I love it, but it is of course premium and proprietary and
Default Gnome apps work well, but you'll need to revert to an earlier version of Gnome Todo/Tasks before it became endeavour, as they had subtasks, but unfortunately removed it with the newer version.