If you're looking to research SHmUP challenge curve, in the Touhou department I think you could start with either Perfect Cherry Blossom or Double Dealing Character.
Aside from being generally agreed upon as good starting points for the Touhou series, these two games I think have a good challenge curve, AND their secondary mechanics don't heavily affect the primary core of the bullet hell aspect. (They're just a free hit/an item multiplier - VS the other games with extra gauges-abilities, or with fancy gimmicks)
I'll believe this will at least allow to better see the patterns and enemy formations in themselves first.
Now (maybe unfortunately) I'm not as well versed in other subgenres of SHmUP, these are lands I've yet to explore.
I know there's a kind that is "more active" than (relatively passively) dodging dense bullet hells. Like for example "Flying Red Barrel" which I've played 10 hours - the core gameplay instead relies on using missiles constantly to erase bullet swarms and get rid of enemies very quick. The more you combo with missiles, the more easily you can recharge your missiles.
I guess you can also turn to the classics to study what ideas they were researching in the dawn of gaming, especially how they had to make games with a limited amount of on-screen elements. In general these games would be fast (perhaps to limit the objects' screentime) and lean on reflexes a bit more than on focus. "Gradius" is what most think of when they hear "retro SHmUP".