Chia seeds have a much higher fiber content than flax: You can make some delicious puddings with thrm, add small amounts to oatmeal, yogurts, smoothies, soup. They need to soak a short time to swell up.
Sweet potatoes, avocado, black beans, chick peas, brown rice, barley, wheatberries…all have soluble fiber.
Add beans slowly to meals and try Beano to prevent gas.
Full Plate Living, which focuses on getting soluble fiber, is on social media and has a PhD nutritionist who answers questions.