What are my options if I want perfectly clean diagonals every time?
If I want a specific diagonal and nothing else, d-pads will often give a cardinal on both press and release as well. Does not matter if it's membrane or microswitch, cross or disk.
Arcade sticks also use four microswitches, so unless there's a special configuration that helps I doubt they're any different.
Keyboards and leverless involve simultaneous press and release with two fingers, which is even worse.
Joysticks are technically capable, but between the small size and total lack of tactility they're harder to be accurate with in general.
Unironically best solution I can think of is numpad.