As I suspected, one of my good friends got to meet Mr. Rush at least once in the past few years, and one of his friends/biz associates was much more than a casual acquaintance (that gentleman is a well-known expert on space and ocean science and has an engineering education from an awesome school-- we'll leave it at that).
If I recall correctly...
The CF tube/Ti pressure capsule concept was at least in part chosen because a larger space for more people could be possible at a weight that is more "enabling."
The composite concept was attempted previously: https://en.wikipedia.org/wiki/DeepFlight_Challenger
See also:
https://en.wikipedia.org/wiki/Hawkes_Ocean_Technologies
https://en.wikipedia.org/wiki/Steve_Fossett
https://www.theverge.com/2014/12/13/7387153/virgin-oceanic-plan-to-explore-ocean-depths-scrapped
The Titan CF tube/Ti pressure capsule was tested at Woods Hole and passed at least at some point. Perhaps more than once.
I suspect that computer modeling was done, material scientists and a variety of outside engineers were involved, and if I recall correctly both NASA and Boeing may have been involved at least at some point.
The company has been around since ~2009, and has apparently built more than one submersible (you can see at least one such vessel which I think they named Cyclops in some of the video coverage).
Apparently the Titan vessel that was lost has made about 4 dozen various dives at various depths, and perhaps we will learn more about its history of testing, et cetera.
Apparently one hull exhibited signs of eminent failure in a test at some depth. It was taken out of service and examined and learnings were applied to the next one.
Based on what I have stitched together from public reports, in my opinion it is not appropriate for the Titan vessel to be labeled a high school science project, or a DIY project.
Ocean tourism in submersibles is apparently an "activity of the future." For example folks might want to take a look at the Triton web site. Cameron is apparently a partner in that venture. They have a wide range of subs and might be considered a competitor of sorts to the company run by the departed Mr. Rush.
I hope that more will be learned about what happened for the benefit of family and friends of those lost, and folks working in the field of ocean exploration.