im not even joking when i say in my whole 3 years of playing genshin, i have only R5ed only one craftable, the Prospec Shovel for Flins. I know you can techincally earn them with exploration rewards, but once those run out, ur screwed.
And worse? There are now 3 variants of billets that are locked to specific bosses
Even worse? Dream solvents are just as rare as ever, and it costs 2 dream solvent to convert a billet to your liking

Its tedious system, already hampered by the fact you have only a few chances per week for it to drop. My personal suggest would it be to be available on the starglitter store every month but idk, just give them a bit effiencicy to drop
Hoyo really needs have more efficient ways to earn billets rather than locking them behind weekly bosses.