I have 3 kinds of mental vision.
One, which some people call "phosphenes", is the ability to paint mental images on the field of vision that corresponds mentally to the signals I get from my retina. I can only see these kinds of images with my eyes closed and shielded from light. My ability to do this has waxed and waned over the years, and has never been good enough to create photographic imagery. At one point, I lost the ability to generate any colour, and I trained until I got colour back.
Two, the ability which seems to be called "phantasia". This is photographic mental imagery that runs on a separate channel from my visual channel, so I can consult this channel with my eyes open, and see things like people's faces. I couldn't do this at all a number of years ago, and was unaware that I was missing this ability. Then I discovered I had aphantasia, and I did some training which got me up to the level that the OP calls "hypophantasia", or maybe a bit better than that. I think that ability may have waned over the last few years since I last trained up.
The "afterimage test" is physically working at the phosphene level. It does not provide an accurate subjective experience of phantasia.
Three, the ability to visualize 3D geometric objects. It works great for wireframe polyhedra, and also works for maps that I hold in my head and use to navigate while driving. This ability has always been very strong, and I originally mistook this ability for phantasia. This is the ability I would use for your equilateral triangle example. I can position 3D objects in space relative to my body, rotate them, etc. Because I am strong at this ability, it is my job to efficiently pack luggage into the vehicle for camping trips. My partner is bad at this, but is much better at phantasia, and is a visual artist who can draw realistic faces.
What I wrote is based on personal experience. I am not selling anything. I know from internet forums that different people have different wiring in their brains. Some people have subjective experiences that other people have never encountered. It is impossible to prove the existence of first-person subjective experiences to a determined skeptic. The best you can do is collect field reports from a lot of people, compare them, find patterns, then make your own conclusions. This is called phenomenology.