What is causing my extreme slow transit constipation? It's just getting worse and worse.

It's absolutely 100% not a diet/water/exercise/lack of laxatives issue so don't spew that garbage ITT please. I've tried all of those for over 10 years.

It's most likely an issue with my digestive tract not getting the proper electrical/chemical signal to move food/waste through my digestive tract.

Now, I know the intestinal tract uses:
-serotonin systems
-acetylcholine systems
-nitric oxide systems
-adrenal systems
to stimulate itself to move.

One of those is extremely messed up in me.
I'm doubting it's a serotonin issue because I've taken 5ht agonists to move the colon and it barely helped at all and was pretty much worthless. I've also taken 5htp to create more serotonin, which didn't work, but it could also be that

I'm doubting it's an acetylcholine issue because I've taken acetylcholine precursors and acetylcholinerase inhibitors in high doses and they didn't work at all.

Health history:
-3 month benzodiazapine addiction which really permanently messed me up
-I don't eat gluten/dairy products because they make me sick, I can't even eat fiber anymore because it constipates me, I'm mostly eating potatoes/meat and sometimes veggies

What other biochemical systems are responsible for moving the intestinal tract?
Does anyone have any other information on these systems and what substances I could take to alleviate this issue?

Thanks desu.