Arrow Flash! It's simplistic, short, easy to get the hang of and has some built-in cheats that can give you a light headstart in the first few stages.
MUSHA/Full metal fighter Ellinor(what a dumb name lol, I'm just picturing whitey from eight crazy nights piloting a mech) Is also super accessible and forgiving on top of also being very simplistic, while not skipping out in having some of the most insane music and visual design on the system!
Insector X is a good introduction to "trial and error" shoot em ups, having checkpoints and levels that while simplistic, encourage you to learn enemy and obstacle patterns so you can overcome the stages quicker and quicker on subsequent playthroughs, and on the continue screen all you need to do is press up-left on the dpad and c and you get an extra continue, and afaik this works every time you game over so like Musha, it's also a very forgiving experience!
Bari-Arm is the best shmup on the sega cd imo, as it not only has amazing graphics and music, but also has very chill easy-to-get-the-hang-of gameplay, and moreso than any other previous game has AN INSANE ammount of cheats and accessibility trinkets to mess around with!
