I managed to setup a layour rule to show the subform when the field has some values.
Even though the subform as well as its columns are set to mandatory, they are not required after the field value changes.

Is there any way I can force the user to fill in those fields once he changes the values of Field No 1?
I'm using the canvas and that could be the main issue -> If I were to click "EDIT" and then change field no 1 then subform is required.
It is not required if I change the field from the canvas view though.

As a workaround I thought about changing the background of the subfrom to red when field no 1 is not empty - I don't know how to switch it to default color once some entries in the subform are added though.

Thank you.