The documentary is called Doomed! The Untold Story Roger Corman's The Fantastic Four and can be streamed via Amazon Prime Video.
I'd also recommend folks check out Red Letter Media's Best of the Worst video in which they discuss the film. Spoiler: they like it.
This "film" is really just a bunch of episodes of Johnny Sokko edited together into movie length. It was an okay show, although not my personal favorite of it's type. Johnny Sokko is largely derivative of the older Japanese manga/cartoon Tetsujin 28 (known as Gigantor in the the USA and available for streaming via Amazon Prime). If you enjoy these live-action Japanese shows from that era, then also check out Ultraman.
Don't really know how well you've explored CAT III. I only scratched the surface, but in case you'd like some new sleaze from HK, here's my list of CAT III films I've seen.
I still use the term "psychotronic" film/cinema. I'm a Gen-X guy who purchased The Psychotronic Encylopedia of Film back in college.
That book had a pretty significant impact on the "cult" film community back in the pre-internet era by helping to define certain genre films in a more novel way. I'd have to write a much longer post to explain my take on the book, but the TLDR is it changed the way I identify (and appreciate) "cult" genre films.
Completely unrelated, but a couple of years prior to the book being published Troma had a hit in The Toxic Avenger and subsequently began to flood the VHS rental market with intentionally schlocky comedy/horror films. That kind of stuff became a big trend in the mid-late 80s. I only mention this trend (which continues to this day with junk like the Sharknado films) because I've always felt these were fugazi "cult" films. They're basically a mass-market product built around an intentionally and overtly ludicrous premise and their titles are basically click-bait for the masses (e.g. Surf Nazis Must Die). I don't necessarily dislike these films...my point is that these are the types of films that now get labelled "cult." You'd never call one of those films "psychotronic" though, because that definition is going to be narrower and will exclude stuff that is intentionally corny and pseudo-transgressive.
Jim Hosking, the director of The Greasy Strangler , had an insanely weird show on Adult Swim called “Tropical Cop Tales”. I highly recommend it.
https://www.amazon.com/Tropical-Cop-Tales-Season-1/dp/B07NCF9JCY
Andy Milligan is such a fascinating filmmaker. Very low- to no-budget, but nonetheless compelling in a weird almost indefinable way. The book THE GHASTLY ONE: THE SEX-GORE NETHERWORLD OF ANDY MILLIGAN by Jimmy McDonaugh (who also wrote a great bio/filmography of Russ Meyer) got me hooked. Even if you don't like his films, it's a gripping read.
Amazon.com: Hybrid Vigor eBook: Schneider, John Lee: Kindle Store
Here's one you might like - a giant hybrid croc menacing the Everglades.
The 2014 cut. They discovered loads of footage thought lost so it's much more than just a different cut; entire scenes have completely different takes. The whole narrative makes much more sense.
https://www.amazon.com/Nightbreed-Directors-Cut-Craig-Sheffer/dp/B00OPX4NZG/ref=sr_1_3
Huge fan, discovered him from this great book:
https://www.amazon.ca/Yakuza-Movie-Book-Japanese-Gangster/dp/1880656760
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Have only seen Shogun's Joy of Torture, two of the Abashiri Prison movies, and Horrors of Malformed Men. I'm lucky enough to live near a DVD rental spot with lots of underground movies but they don't have this one.
I found this by accidentally clicking the new tab on r/all so I'm not familiar with this sub and thus this may not be helpful, but it's on Amazon for streaming.
https://www.amazon.com/Necropolis-LeeAnne-Baker/dp/B01I48Q73S
How about this?
I don't own it, not yet anyway, so I can't vouch for it's video quality.