I think the largest one is Wiggle, which is going into some European form of bankruptcy.

If you wont miss the extra $2K, getting the SL8 is probably worth it. Easier access to spares, easier shop service if you need it. Proven performance etc. and you will get some of that back on resale if you swap bikes every couple of years.

Regarding your concerns around fitting other bars and stems. This bike uses a 1 1/2” upper bearing, so you should be able to swap it out for headsets from Deda and FSA and use systems compatible with their designs. You will lose the shaped transition spacer. Integrated bars tend to come with fit kits including their own spacers. Higher end Chinese ones like the BigRock Skyline II even include their own C ring.

RE: 3D printing. Some people release their 3d printed designs on sites like printables. I personally do not, as I don’t want to offer support for the design or deal with people printing the designs in materials that they aren’t designed for and then telling me the design doesn’t work.

Without commenting on the YouTubers, I think the best-value frame that has evidence of aero testing in development is probably the Elilee Blize (~$1700 shipped USD including bar).

Wheels, probably Hunt’s aerodynamicist line or the Zipp 303S? I cant think of a Chinese offering that has been developed in that manner.

Bars, EXS?

Yikes, first failure I've seen of this crank. Its a shame because its one of the only 29mm spindle cranksets on aliexpress that's reasonably lightweight.

edit: looks like a similar failure linked here: https://www.reddit.com/r/bicycling/comments/16p795v/dont_buy_zrace_rx_cranks_i_guess/

This is pretty much exactly what I’m looking for as well. Closest I could find is Farsports, which have a 58 depth, 24mm internal wheel with carbon spokes that use their in house ratchet hub (36 or 54).