Welcome to the forum, it can be tricky tiling over the cables. Lots of care is needed. And you can get a gizmo you attach the wires to and it bleeps or something if they break, so you know the exact spot and don't need to pull up tiles. There are also repair kits for heating too. So don't worry too much, just take lots of care.
I'd still use the round notch trowel, too.