Oof, glad you managed to retain your sense of humor through the experience, Stephen Sovie!
I’m a vegan and if I may be so bold, I’d recommend you avoid processed foods containing added fiber and focus on whole foods instead. If you have IBS then avoiding gluten would probably help, too.
One excellent way to get plenty of veggies and fiber is to make delicious soups. I tend to have a blended base to which I’ll then add chickpeas (or lentils) and some other veggie for texture (spinach or chopped up green beans). Use good quality extra virgin olive oil and drizzle some more after serving.
You can experiment depending on the veggies you prefer and add herbs and spices for taste. This kind of meal is very filling and satisfying, and you can even add short grain brown rice, or brown basmati rice for some extra oomph.
Probiotics could also be a helpful addition to your diet if you like them. Rather than expensive pills that don’t always do much, think of fermented foods like kimchi or sauerkraut that can easily be mixed with cubed avocado and grated carrots for example.
Before you know it, you’ll be eating plenty of fiber. For breakfast, how about good homemade oatmeal (easily customizable by adding nuts and seeds, chopped up dried fruit, grated apple, banana slices, or fresh berries) or birchermuesli instead of the disgusting sawdust you describe?
One word of warning: always make sure you purchase toilet paper in bulk and don’t be alarmed by changes in your bathroom habits.
Lastly, do drink enough water as being dehydrated is key to a happy digestion.
You can’t go wrong with ditching processed foods but eating a more natural diet does involve some amount of prep work and cooking, both of which can be relaxing and enjoyable. Plus the internet is a treasure trove of recipes for those days you have no idea what to make with the veggies in the fridge.
Bon appétit !