Disclaimer: I'm autistic myself, I've done some volunteer work with autistic children and it's a subject I feel strongly about in general. That said I have no formal qualifications pertaining to this, and I most certainly am not claiming to speak on behalf of all autistic/neuro-divergent folks.
I appreciate that it isn't your intent but you're taking some harmful stereotypes of autistic people and using them to produce a villain. And it's making my skin crawl. So I'll get to the more general advice in a moment, but first I have to highlight some pretty egreious missteps.
He doesn’t react like most people do to emotional triggers such as seeing something tragic happen (as an adult, he works on the docks. I may have him witness a fight where someone dies and have him kind of shrug it off; just thought of this).
What you're doing here is "othering" Tafani - discouraging the reader from empathizing with him by portraying him as something alien (What a monster! Dave died horribly in front of him and he didn't react!), as a literary tool it's effective albeit crude. But it's also an extremely offensive stereotype.
Why? Say it with me, nice and slow:
Autistic. People. Are. Not. Robots.
A common misconception is that autistic folks "don't react", don't display (or even feel) emotions like NT people do. But it is more accuracte to say that the indicators of their emotional state/responses to events are different and are therefore missed by Neuro-Typical (NT) people in the same way that Neuro-Atypical (NAT) people miss those same things in NTs It's not a perfect analogy but the best way I can think of describe it is if NT body-language, facial expression, tone of voice etc are "English" then for a NAT they're "German". Neither group can understand the other easily without learning a bit of the respective "language", and because the vast, vast majority of human society is built around "English" in these terms it's typically the NAT who attempt to understand the NT (to varying degrees of success) and this drives the misconception.
So if you're going to have Tafani be ostracized by his coworkers for his seeming lack of reaction to the coworker's death that's one thing - but I think you'd have to go some way to indicate to the reader that they are affected, just differently and presenting differently as a result.
Something happens when he tries to interact with the MCs. He’s built them up so much in his head that he makes a bad approach, it escalates, and turns violent. He escapes the situation, and the MCs become “enemies” that he convinces himself need to be “dealt with” rather than seeing how his actions could have been different and re-approaching to a) apologize, and b) establish good relations.
From how you've described this Tafani's neuro-divergence makes him awkward when meeting the main characters (so far, so reasonable), but then you parlay this into him being an idiot who (despite an established history of poor quality social interactions) decides that means the MCs are enemies and should be elminated (ugh, NAT people spent decades being mislabelled as mentally deficient, "slow", "retarded" etc) and then you suggest that he's a bad guy who should just apologize and "establish good relations" (ugh again. Distinct "have you tried being normal?" vibes). Honestly, this reads like the neuro-divergant version of the gay villain whose start of darkness is when they come-on to the heterosexual hero who turns them down, and I hope you'd see why that's a bad thing.
There's nothing saying you can't have a NAT antagonist, they're no more immune to being assholes than NT people are, but making their neuro-divergence the reason for them being the antagonist is the problem.
Right, on to more positive advice..
Anyhow, if anyone has any tips on portraying the internal monologue of a neurodivergent person
You've already hit on one of the major things - the "always going" factor, this is surprisingly hard to describe simply because I've never known anything different (indeed I was probably in my late twenties when I realised that this wasn't the case for everyone) It's going to be difficult to portray in writing - you can't simply relay it because it would be so pervasive as to drown out the actual story. And for me at least it jumps around alot (a good anaology would be the way a modern operating system gives tiny slices to many disparate applications/processes), for example while I've been writing this post it's been covering writing this post, self-moderating the tone of the post, the music I'm listening to (should I change tracks?), what time I need to leave work to accomplish my errands, what should I have for tea, a couple of story ideas, working, why did the Roomba get stuck this morning?, plans for a friend's birthday next week, weather and the chances of cleaning the car this weekend. Honestly, I could go on, but hopefully you get the idea. There's a great deal of it - and much of it isn't going to be relevant to the story so you don't really want to include it. My suggestion for how to portray it would be to focus on the effects it has on his life (he may seem distracted, have difficulty relaxing, he may make a comment unrelated to current external circumstances but that's been prominent in the inner monologue etc) and only dip into it when you need to show something relevant (analysing a recent interaction, preparing for a future one, etc)
I’m not quiet in social interactions. Indeed, I often think (here the social anxiety) that people just want me to shut up.
The key to removing this blocker for you is to identify the rationale behind Tafani's quietness in these situations. Is he like me who can quiet in social situations either because social situations are often sensorily overwhelming or because I don't know the people well enough and therefore I don't have enough a) working knowledge of them to avoid putting my foot in my mouth and b) spare mental bandwith to keep up with interpreting what they are doing AND produce my own responses.
I have a lot of friends, and know I can rely on them for support when I need it.
I’m married, and so don’t spend my life alone. I’ve been married long enough that I can’t even imagine what it must be like to live like that.
That they're neuro-divergent isn't particularly relevant here. Assuming you don't want the character to have friends or a partner then just like any other character in that situation you can either have them be someone who is comfortable with their own company or lonely and wishing they had more connections.