Long term constipation solution?

I know this question has been asked to death here and I have read alot of suggestions. I've tried alot, I hope not everything.

I've been on a stable dose of 80 mg for a while and constipation has been an issue since I started methadone 8 years ago but the last 2 have been pure hell on my g.i. system. At one point i ended up in the ER for dehydration because i threw up everytime i ate due to severe constipation, they were no help, told me to take mag citrate and gave me zofran ( which btw causes constipation). Miralax worked for a while, until it didn't. It started giving me these bizarrely thin pencil poops that never left me feeling like I got it all out.

I've tried mannnny Amazon "cleanses" and sure they clean me out but the stomach cramping is so intense that I have thrown up and also had pretty severe cravings to use from the pain. Senna, biscoydyl etc kinnnd of work but do give me unpredictable bouts of diarrhea. Like don't trust a fart.

I know I don't drink enough water or eat enough veggies so I've been implementing that as well as kombucha and kefir but it hasn't been long enough.

I really want to start tapering and get off this shit but in the meantime I truly can't live this way. The only time I feel "good" is on days I don't really eat and that's not sustainable for me, I work a physically demanding job and not eating makes me really tired and weak. I see so much contradicting information like is fiber good or bad? What prescriptions have helped you which haven't?