Incomplete evacuation / dyssynergic defecation??

I have IBS for 4 years now and my DAILY symptoms are mostly excessive gas (often trapped) and incomplete evacuation / constipation. Tried and tested for everything, nothing helps.

I can poop every day (multiple times) but can never fully empty. I sometimes sit on the toilet for 30-40mins because the stool comes in waves (stool - gas - stool - gas).

I was thinking IMO, but methane test is negative (3x).

I have an anorectal manometry next week to see if i have dyssynergic defecation. Does anyone have this or was able to successfully treat it?

Or does anybody have similar problems and knows other possible causes or successful treatments?

This is for more background on my symptoms and what I have tried already: https://www.reddit.com/r/ibs/s/BcSKsCMkyP

Would really appreciate any help. I don‘t know what to do anymore. ❤️‍🩹