getting blue crow the second and golden wings are pretty skill-based

Sounds neat, gotta look at what those are for.

listen to blue crow in stage 1 before shooting him down, shoot his guns off while barely damaging the main body in stage 2, get 3 or more stars in every stage and don't use any continues before his stage 5 encounter to fight blue crow the second and get that achievement

golden wings is obtained by beating the game without using any continues

Oh - well, a regular 1cc achievement is fairly standard. Nothing special.

They arent grindy at all.
5 for clearing stages
1 for learning the basics of rocket chaining
1 for having a handle on the basics of scoring
1 for a gimmick against the first target in the game
1 for the 1cc
3 for spawning and defeating hidden encounters
3 for ending progression, all 18 of which require that you route your encounters in different ways that you would be hard pressed to figure out without a guide
1 for having all other 15
Aside from a few specifics that they added themselves, all of the achievements are derivative from what's tracked in the original Japanese before it ever gets localized and put onto steam.
Be realistic, an STG game you're expected to play multiple times simply for score attack or for the sake of improving. Aside from 1cc, secrets, no-hit runs, and challenge runs there really isnt much room for achievement variety to begin with.

18 endings is what I'm talking about, though. Feels pretty superfluous for an STG.

Most STG titles have a linear story as is, maybe having a special good/bad end depending on performance.
FRB deliberately was designed with branching in mind, most character encounters are resolved in one of two/three different ways; Peat for example can be spared, shot down, or shot down before hes done talking in stage 1, several bosses have different consequences for shooting out specific parts of their vehicle, etc.
It's to keep you from flowcharting your way through the game with the first thing that works, at least until you hunker down for score attack and have a route in mind.