Thistlegorm is about 32 to the seabed. Upper parts of the wreck are much higher placing it comfortably in reach of the experienced recreational diver.
Interestingly the more recently discovered "must see" wreck for liveaboard divers is the Rosalie Muller. When I saw her 3 years ago she was so undisturbed that pots were still hanging up over the galley stove! She sits upright in 42/43m with main decks at about 32m. That is Definitely cutting it fine on air for time duration, and divers considering this should at least have a deep diver speciality training, and I'd recommend a redundant air source (pony cylinder).
The Ras Mohammed caves in places are known to go down to 90m with entrances at 30m, but cave diving itself is a very specialist field. There are some shallower caves and caverns too.
is your friend
Utlimately and diving beyond 40m should be considered as tech diver only, but that would explain why you saw divers with loads of tanks. They would need them, some would contain special gas mixes suited to the depths, and above all their training would allow them to do such dives, in a competent manner.
Solo diving around the world is frowned upon. It's something that would need a lot of redundant kit so as to cover failures. The recreational diver is unlikely to have such kit.
There will always be the odd idiot who exceeds or ignores the guidance. Insurance, dive guides and most importantly dive buddies shouldn't be pressurised or expect to have to pick up the pieces for their actions.