It appears that the composite outfit that helped design and eventually constructed the Titan's pressure capsule also help with design and constructed the DeepFlight Challenger.
https://www.compositesworld.com/articles/deepsea-submersible-incorporates-composite-pressure-capsule
"HOT subcontractor Spencer Composites Corp. (Sacramento, Calif.) was given the task of designing and fabricating the DeepFlight Challenger’s inner carbon/epoxy composite pressure capsule. A filament-wound cylinder, the capsule has a thick glass viewing dome on one end and a titanium foot dome on the other. The domes would be attached to the composite laminate via bonded titanium rings. The tube wall thickness had to be sufficient to resist a biaxial stress field exerted by the deep ocean pressure of approximately 16,000 psi/1,100 bar, with a 1.5 factor of safety. According to Spencer Composites’ principal Brian Spencer, all other design factors, which included water ingress, temperature performance and ability to withstand handling loads in and out of the water, were insignificant compared to the external pressure load."
"Spencer built a number of half-scale tubes to test several designs, including different hoop-to-axial fiber ratios, variations of fiber type and fibers of different moduli in the laminate. Tests were conducted at Pennsylvania State University’s (State College, Pa.) test laboratory, one of a handful in the U.S. able to generate the necessary compressive stress loads. Results were compared to the finite element model and anticipated failure modes. Spencer reports that the subscale samples withstood a maximum fiber compressive stress of more than 125 ksi/1,250 MPa and exhibited compressive strain capability that exceeded 0.48 percent."
The DeepFlight Challenger was designed by HOT:
https://en.wikipedia.org/wiki/DeepFlight_Challenger
https://en.wikipedia.org/wiki/Hawkes_Ocean_Technologies
It was designed to go to the Challenger Deep... about 2.5x deeper than the Titanic wreck's depth.
The idea was scrapped, and it appears at least in part because test results suggested that it could only dive to Challenger Deep depth once, and the sponsor wanted a vessel that could go multiple times with tourists. That was way back about a decade ago.
HOT apparently spent quite a few years (decade) working on engineering and material science challenges through a number of prototypes and perhaps even products they sold. I suspect that their subcontractor and others were involved as well.
We may or may not learn that the design, and/or construction, and/or testing of the Titan's pressure capsule was "informed" by learnings from a decade or more ago.
And of course the design challenges for the Marianas trench are different from those for a vessel designed for 4k meters.
I suspect that there is a relatively small community of expert folks in the super deep submersible space.
Some of them were likely at this 2019 conference:
https://oceanplanetconference.com/
I know for sure that Stockton Rush was there. And of course it is likely that the keynote speaker, Dr. Sylvia A. Earle, attended.
Interestingly it appears she was once married to the founder of HOT.
Again, condolences to those that lost family members/friends.