Worth noting, you can get a diagnosis and then choose who you do or don't disclose it to, including doctors to whom it is not relevant.

I have heard that autism is a sufficient reason to be denied immigration to some countries, but I haven't looked up the details myself.

EDIT: Internet says Australia, New Zealand, and Canada are among them. https://en.wikipedia.org/wiki/Discrimination_against_autistic_people#Immigration

If you already have an ADHD diagnosis, there probably is no point seeking an autism diagnosis. Being that there's no medical treatment, and what disability protections there are would be covered by the ADHD diagnosis, I suspect you'd be best off trying coping techniques on your own. Which is mercifully quite easy to do.

The biggest game-changer for me was considering that I might actually have sensory sensitivities. I always assumed I didn't have many, if any, but it turns out a huge chunk of my day-to-day anxiety was due to light and sound overstimulation.

You absolutely should seek help when you feel like something is off. I feel like the recognition of ADHD is very location and provider dependent and it might be you live in a place where it’s harder to have that diagnosis taken seriously. There might still be some relaxed guidelines from COVID that allows telehealth doctors to diagnose and treat ADHD so that might be another option. It really sucks to not have supportive doctors and I’m sorry you’re going through that. As long as you have the diagnosis, your insurance should still pay for meds/counseling/treatment, so there’s that.

Also, there can be a lot of overlap between ADHD and autism, so for practical purposes there might not be much use in getting an official diagnosis for autism. Doctors view a diagnosis as a first step towards treatment and if a new diagnosis wouldn’t change the treatment plan, they might not deem it as very valuable. Which might make you feel like you’re not being listened to or that your issues aren’t getting addressed or recognized. Please don’t let this prevent you from seeking therapy. Therapy is pretty open-ended and if you find a good therapist, they can gear it to your actual issues you’re facing no matter the official diagnosis. It sounds like you could use more people who have your back.

In the United States, do not call the police, do not do it, their goal is to destroy your life, full stop.

I ended up with like half an autism diagnosis a bit over ten years ago. (Basically, I saw someone for other reasons and they said "Um... I'm pretty sure you're autistic, you should go talk to these people for a proper diagnosis" and I never did.) Occasionally the idea resurfaces (and is again at the moment, to some degree, because I'm having problems at work that are surely neurodivergence related) and I end up dropping it. Mostly, as far as I can tell, as an adult who is able to live independently, maintain employment, and isn't going to return to education, there really aren't many/any resources out there, so it feels a bit pointless. Some people do get a lot of benefit from the confirmation/certainty that comes with a diagnosis, so you may feel a diagnosis is worthwhile for you, even if it doesn't get you access to any concrete resources. I can't decide if I'm one of those people or not, to be honest.

Now, there are concrete downsides to diagnosis--some countries will use an autism diagnosis as grounds to deny a visa; in the US it's not an unrealistic worry that it'll make accessing medical transition harder if you're trans; I have a friend who has come down on the side of "no official diagnosis" for fear it could jeopardise his access to ADHD meds in the future. (I picked up an ADHD diagnosis a couple years ago -- I'd been taking meds for anxiety and switch psychiatrists and they were like "Umm... I'm not saying your not anxious, but you're actually describing ADHD". I suspect my brain lies in the autism/ADHD uncanny valley. I mention this as a lead in to say that I don't share my friend's fear, but it's also not an unrealistic fear.)

i cannot speak about an autism diagnosis and if it will be helpful or not. I did do a little searching and found these results that are united states based. I realize this a global community and the site might not be as useful as your geographically relevant resources but maybe it could help. https://search.usa.gov/search?affiliate=iacc&query=resources+for+adults