bullet hell shooters with simple scoring?

[Despatche]

bit of a bump but... ketsui mr. stoic.

[spadgy]

Another vote for original Mushi.

And while executing the technique can be pretty complex if you play it at the high end, the mechanics of scoring in the Psyvariar games are pretty simple to get one's head around; might be good for an accessible scoring contest.

[gs68]

rRootage on Normal mode. Just hold down the fire button, don't get hit or fire bombs (if you can), get close to the boss to do extra damage (which will result in higher time bonuses).

[spadgy]

Good call on rRootage. Kenta Cho/ABA Games in general, perhaps? Free, easy to get hold of, and easy to get into.