Commercial pectin products often come from processed apple skins, and some other fruits but mostly from the skins anyway, so you’re spot on

No idea if peppers will replicate the stock texture of celery but I imagine it can’t matter too much. You probably have to be extremely sensitive and test a lot of samples to tell the difference for that kind of subtle texture

I personally like celery so I don’t share your problem but I rely more on boiled potatoes anyway in my soups/stews for the starch as a mild thickener. I also love potatoes (: