I've had or have MENS convertible pants with partial or full elasticized waistbands W/ INTERIOR DRAWSTRINGS some that came also with belts(that I promptly removed with the belt loops if they had them but not for your reasons but to save wt and bulk, I agree though if you pack's hip belt lays in the same place chaffing can occur, sounds weird but the rare few times I recognized waist chaffing beginning I tucked in my tee or baselayer under my belt)) made by Ex Officio, Roundfield(maybe Roundtree?)&York, Columbia and TNF. Don't have any of them in front of me now so you have to look up the models. Problem with all these companies is that styles and versions w/ this drawstring feature change based on current production.
Might look at some of the fishing convertibles. Your chafing issue may be solved by wearing partial elasticized waistbands in itself. Then, you might have another issue like your convertibles not staying up. FWIW I like the elasticized waistband with drawstring whether convertible or non convertible pants on longer hikes as my wt and waist size fluctuates downward allowing to compensate without them falling down.