There are two axes here! Phantasia does not imply involuntary imagining! I don't imagine/picture involuntarily, and for many many years I didn't understand that most people do involuntarily imagine what they hear. Forbidden-while-eating topics are no longer a mystery to me.

Interesting.

My own mental model (sigh...) is that these are all roughly related to (if not literal types of) the crossover/cross-polination (sigh...) that manifests in synesthesia.

I think synesthesia is definitionally involuntary, but your statement did make me go search "voluntary synesthesia", though I'm not done reading results yet.

I would guess it's a lot harder to identify people who can electively conjure anything akin to what synesthetes experience. It probably feels less distressing or noteworthy, happens less often, and would be a lot harder to study...

While I have the conscious ability to create both images and entire 3d animated systems in my head, it also comes with my subconscious flashing images at me, from animal forms through to mandalas (sometimes combined with buzzing noises), having extremely intrusive visual memories that can be triggered unexpectedly off a host of random things, and experiencing fireworks whenever I have a migraine. I think I might be sitting in a tail somewhere too. Hi, how's your tail doing? This one is very brightly coloured but tends to move around a lot.

This is similar to a conversation I had with my wife about inner dialog. I had always considered that one's inner dialog was literally conscious thought until I learned that some don't have that at all.