Cosmo Dreamer's Extra Mode is undoubtedly based on Touhou's Extra Mode. There is no difficulty selection. The good news is the pattern is only as hard as Expert / Hard. You don't need to be Nightmare / Lunatic good to beat it.
We know the deal: Bomb immunity, small boss hitbox, and dozens of phases to fight through with minimal extends! I recommend the B focus shot. A bomb is mainly for scoring.
Overall it's a fun romp. I find the stage parts more interesting since there are ground enemies compared to Touhou's.
As usual, the thing I'd criticize the most is it's only using royalty free music. It's 100% okay, but it hits awkwardly when you've just heard this song when you were watching a random Japanese Youtube video that so happens also using the same royalty free musics.