Humans began to rapidly accumulate technological knowledge 600k years ago

That's wild - I am clearly the first sort, because I cannot imagine what "a smooth stream of thought" would even be if it were not expressed in words.

It is like your motor control thoughts, you don't think let me pickup this spoon you just pick it up, when given an equation you do not think go through the steps if x - 9 = 22, you just say x is 31 yet you have skipped in your mind there exists a base mental representation of these things some call it a small world model of a large world model encoded in an abstract knowledge representation sort of like a compilers abstract syntax tree and this representation is sort of universal in that you can take it from someone and give it to another person and they will automatically know provided that they have the requisite properties of the world model in which it exists for instance a thought about the symmetric group s3 could be transferred provided that the requisite structure for groups( just the definition, and concept of rotation exists which already exists by default in mammals).

For what it's worth, I used to be certain that I thought in words too. Then I moved to a different country and used a second language often enough that I sometimes think in it. I then realized that there are periods where I think in neither my native language nor in my second language, even when I'm thinking. YMMV of course.

Do you find your domain of thought shifts when you do not think with language? For example, do you feel you are able to do complex reasoning without language?

I don't think in words unless I am speaking, writing, or modeling a conversational interaction. The vast majority of my thoughts are in the form of sense impressions, motor sequences, visualization, or wordless intuition.

I'm sure you think that way too, you probably just layer a narrative over it. The sibling comment about picking up a spoon is an example I sometimes use - see yourself walk to the kitchen, move your hand to open the drawer, pick up a spoon, pour the tea, scoop the sugar. I can describe them but it's not natively linguistic to me.

I'm hell at rearranging furniture or putting together an engine, not so good at positive self talk.

It’s a process of serialization. I frequently have thoughts or ideas that take me a while to express in words. Being stuck to words seems so limiting, slow, and linear that I have a hard time believing it; surely there are more fundamental mental processes generating the words and the monologue is just a serialization of thought? Right?

I usually dream in English - but I don't speak English. But I understand everything thanks to my dream's subtitles.

This describes my experience nicely too, although I didn't quite realize it until reading this. I was thinking I just thought in words and had a tendency to mix up the bits of various languages I know some words from, but occasionally I have some thought, then for one reason or another become conscious of the thought and get confused which language I was thinking in. Sometimes this even manifests as trying to mix languages (eg at one point I used to struggle to not sprinkle in french and farsi words into normal english, despite speaking english at a native level and only knowing the basics of the other two).

I've always thought of my mental model as an endless conversation with myself, but I think the more fitting description would be a "smooth" series of thoughts which only materialize into language when I explicitly focus on those thoughts as their own thing.

I do also think visually for things that have a visual component though.

That's an interesting perspective, and I think you are on to something.

I have some experience with a meditative/dissociative state in which that monologue - which I think of as the "narrator process" - can be observed as just one of many mental subsystems, neither containing the whole of my consciousness nor acting as the agent of my will. The narrator merely describes the feelings which arise in other mental components and arranges them, along with the actions I take, into some plausible linear causal sequence.

Minds differ in many ways, and perhaps one way your mind and mine differ is that words flow quickly for me and do not feel slow or limiting; so I suppose I am easily fooled into perceiving that narrative as the medium of thought in itself. It had not occurred to me to describe the activities of the other mental subsystems as thoughts, but why shouldn't they be? And now I have a better guess at what it might be like to experience the world in a different way. Thank you!