Warming the ear does help a lot! Sophie’s ears always bleed best after she’s been running around the house like a maniac and they’re hot. And they really do bleed easier the more you poke them! I’ve seen from many places that cats don’t have many nerves on the edge of their ears, but I’m not sure if that also applies to the whole ear, so I personally would be hesitant to try.
You don’t want to hit the vein, as that tends to hurt more, but just along the outer edge. It may be helpful to try pricking different places along the edge of the ear if you haven’t tried that - the “sweet spot” you see in diagrams is a good place, but Sophie’s ears also bleed well right at the very top. And you may find that one ear bleeds better than the other too. The left is always easier to get blood from than the right one for me