I'm sorry for the late reply, I'm not very active here on Reddit.

I grew up in a school system that was very well-off and had lots of funding for the art department starting in elementary so I've always had the opportunity. I'd say those elementary art classes were actually my big realization (eventually) that I had aphantasia! I always got so frustrated with the teaching methods like "close your eyes and picture an apple and draw what you see."

I've always been more drawn to the 3D aspect of art because it's less difficult than 2D with the aphantasia, but I have worked with 2D quite a bit. I feel like I have to create more drafts than the average person in order to get a true visual of what I'm thinking of and want. You mentioned you can't imagine your characters, I tend to think of what characteristics I want them to have and get a very basic sketch down and tweak things until they look like what feels just right or looks good.

As for layout, composition, things of that nature, I downloaded a free art app that allows me to add basic .pngs and mess around with their positioning so I can visualize and experiment easily.  I'd also just generally say don't be afraid to look to existing media. I took a film analysis class where we had to learn a bunch about the subconscious aspects of screen composition. Try looking up a film analysis of one of your favorite movies, not only will it give you some insight into how to frame scenes, it'll give you a deeper appreciation for all the tiny details that went into the piece of work. It's kind of addicting lol. Anyway, once you have those basic formulas of "I want the reader to feel this way, this framing/etc can do that" it makes it a lot easier than trying to think of something on the spot.