As you said yourself:
He tried many services and applications as known(todoist, any.do, etc.), not really, but perfect none found.
And so do all who are looking for the perfect planner. Probably this is not there and the search stops on a more or less acceptable option. And this cell will soon be chock-full if you do not offer something over and above the present.
An example from practice. Asked the investor with the idea: task-tracker + daily planner. Described as it it seems that there must be a concept. Needed a team and the person who organizes the development. Requirements for product Manager, a textbook from Silicon valley: he had this already to develop, there must be failure and success, burning eyes and a full belly.
He was offered a technology tied to AI: it has simplified the interface, had the scale for learning. The product can be output in the b2b segment and b2c. In General, did not. The fire slowly began to melt in front of the investor and the matter was hushed up.
The moral of the story is: that the abundance that is already on the market - it is already there and to do something pile up - success will not add, owing to the exponential distribution. Therefore, if you have a burning desire to create such a product, think towards what will draw in your users: "space" or easy. And then, and it will require something new.
I hope to be useful!