https://www.eonutrition.co.uk/post/thiamine-ttfd-bladder-emptying-disorders-interstitial-cystitisGive that a read and see if it resonates with you. I've been working up on his mega dose thiamine protocol for other challenges and have had good results so far. It seems to be helping my urination frequency a bit as well. Although it sounds like your issue is on the opposite side of the coin - so maybe this isn't in the same ballpark..
https://www.eonutrition.co.uk/post/mega-dose-thiamine-beyond-addressing-deficiency