Sorry you are going through this! Once your GI Dr. tells you there is nothing, they can do you must find the best colon/rectal Dr to go to. The episodes will get worse, and you are causing more damage and scar tissue in your sigmoid.
I remember mine telling me that on the way to the ER for the last diverticulitis episode on I had. He was sad for me and I could hear it in his voice.
I have never taken OTC for fiber. But, I found that consuming it contributed to my episodes because of the diverticula pockets were getting packed and causing the problem. My go to when traveling was pepto in the morning with a big glass of water.
Please find colon surgeon.