Have you tried Amitiza? It's prescription but it was a life saver to me until I developed some food allergies that take away the issue. I was on it for many years. I still sometimes needed a boost of Senna, dulcosate or ducolax (and often combined it with Miralax depending on what meds I was taking) but I went regularly, especially after I learned how to use whatever I needed to supplement the Amitiza.

There's also Linzess but it gave me horrible diarrhea so no go for me with that one.