Yup: only a handful of factories doing most of the mass-market carbon manufacturing. Giant builds for more than just them. ADK Composite Technology builds for a ton of brands, including Look, Fuji, and many more.
Supply chain issues aren't just in the frames. Components are back-ordered at both the OEM and retail levels, especially in the mid-range stuff like Ultegra/GRX800, SRAM Force, and Campagnolo Potenza and Chorus.
Definitely start with a comprehensive fit session. Retul is one fit method, and there are plenty of others that are equally valid and may work better for you. Ask around for the top bike fitters in your area and get that fit dialed. You can then shop by frame geometry.
In terms of bang-for-the-buck, there's some logic to going with a big brand like Giant, Trek, Specialized, or Cannondale. They have better buying power for the frames and components and thus can often get you "more bike" for less cash outlay. That said, they also have a ton of proprietary stuff littered about their lineups that make true customization for fit and preferences more of a challenge.
Canyon is great but you need to be sure the geometry fits you because they are almost 100 percent proprietary stuff these days with stems, handlebars, and seatposts.
Sleeper brands like Scott, Jamis, Bianchi (yup - in the U.S. they're a bit of a sleeper), Willier, BMC, Pinarello, et al offer a lot of upside and may have better model selection right now. I'd steer clear of Cervelo right now as they've had a streak of tolerance issues over the past few years that have many folks concerned (though it seems their new Aspero gravel bike has overcome many of the issues).
Definitely get your order in early. These bikes are definitely moving in the more road bike centered areas of the country. For example, here in the DC area the pickings are slim for anything mid-range or lower, and some of the top-end stuff is limited by component shortages.
As far as used carbon is concerned: caveat emptor. The Pro's Closet does a comprehensive evaluation of everything they sell, but buying from a private party you run the risk of picking up a damaged frame (even if the damage isn't obvious).
Good luck!