Side scrolling as in on the ground (gravity like in Contra, or Metal Slug) or side scrolling as in horizontally scrolling shmup (Gradius, R-Type, etc.)?
Also it's funny you should say that, because I think Touhou is one shmup series that can't really be called "based on memorization".
As I said, Zun makes excellent use of RNG, making sure that you will not get past many bullet patterns unless you have good reaction. This keeps it satisfying on multiple playthroughs. Other then that, there's nothing much to "memorize" in boss fights that you can't figure out during your first encounter. And the difficulty of the games isn't even on the same level as something like Bug Princess on Ultra God Mode, so it's not like you're going to get FLOODED with bullets the way you would in that if you don't know the enemy spawn points and whatnot.
Really, any game with dedicated design can be made easier by memorization. The question is...does the game telegraph it's hazards fairly? Fairly enough that you have a fair shot at dodging them on your first try? I would say yes to both for Touhou.
Characters should not factor into your enjoyment of the gameplay. That being said though: Liking Otomedius but having a problem with Touhou's aesthetic?
Whaaaaaaaaaaaaaaaaaaaat