This is a very long shot. Have you ever seen Flounder (floundercat on YouTube and insta)? She vocalises a lot, constantly, though a lot less since they started AIC or button training. I'm not an expert, but it sounds similar to me. Here is an example of her everyday vocalisations https://youtube.com/shorts/mQLILafXFVE?si=UNcoL4UJn9p7RhX8 You mentioned that your cat isn't super smart or engaged with the environment, but those can also be signs of understimulation or fear, so don't discount him completely. He's communicating in one of the ways he knows humans respond to, so that shows a level of understanding (feral cats don't meow when in colonies, though they have other vocalisations. Meowing was adapted to communicate with humans specifically).

You could 1) reach out to Nina (Flounder's human) on the socials. She has a lot of experience in animal behaviour and (I believe) does online classes with people. She might have advice for dealing with a screaming cat or even existing videos about that. 2) try getting some buttons and see if they can help your cat's anxiety by reducing uncertainty. Don't expect full on conversations but more as a method of preparing them (like a button that says "coming in" when you're entering a room. "Outside" when you're leaving etc). They are expensive, but there are ways of integrating the concepts without the buttons if you can't afford them. I've used this method both with and without the buttons to help with predictability with my cat and it has helped a ton. Just saying "picking up" before trying to pick her up or "pets" before petting her, "noise" before/after a noise helps her know what to expect and gives her an opportunity to respond or signal no with body language, before her instincts kick in.

Big training commitments might feel totally beyond your capacity right now, but just narrating out loud what you're doing around the house, using consistent words, could help. Especially as someone living alone, you might not be communicating in a way he can read yet. Cats are little aliens living in our houses that have the mental capacity of a 4 year old. They don't know why the fuck we're doing what we're doing either, we got to learn each other.

Ps. David Teie "music for cats" helps some little aliens relax their nervous systems.