Whether it's
- Bourgeois (~400/yr)
- Santa Cruz (~600/yr)
- Collings (~1000/yr)
- Breedlove (~2000/yr)
- Larrivee (~6700/yr)
...they all make great guitars but they all use factory production techniques (you can find factory tours of most of them on YouTube, listen to them talk production methods on podcasts, read interviews, etc).
That is, one guy makes necks all day, one guy hammers in frets all day, one guy cuts bridges and fretboards all day, etc. It's just more efficient that way—
division of labor, one of the oldest economic concepts known to mankind.
To me, "boutique" means that you want something made by one person, start to finish.
If that's the case, you need to find a "one man shop" (or one woman shop)—because as soon as they hire a second person, you can bet they're going to divide up tasks to capture those economic benefits of division of labor and specialization.
My choice for a small shop would probably be
Goodall, a three-person family company making <100 guitars a year, but still well known and widely respected. Hard pressed to find one of their guitars under $4k used, but occasionally they do come up. They make both traditional and more modern designs.
Another good choice is
Bob Thompson, also a one-person shop and a skilled builder. Sometimes you can find used guitars in the $3k range but most are $4k+.
As for sustainability, I think Breedlove has done a great job carving out that niche. Since 2017, they only use sustainably harvested wood (plantation, repurposed, natural treefalls, etc) for their Oregon-built models, and they actually
lay out the rules clearly, specifying the material and national origin of every part of each guitar. Starting this summer (2021) they applied the same rules to all their import models as well. I am not aware of any other builder that has made that commitment to both sustainability and clear disclosure of sourcing.