the FDA allows it to be called fiber, but it acts differently in the body to the soluble and insoluble fiber that most people are familiar with
IMO, it's a mistake to treat fiber as a single, fungible food component, rather than as a family of different substances with different health effects
if you're looking for "easy fiber intake", there is much more and stronger evidence for psyllium-based soluble fiber, widely available in cheap and convenient drinks like metamucil (I prefer formulations without the dye and maltodextrin, but that's getting into the weeds)
So basically toss a shovel of potato starch into your protein shake and be done with it. It's flavored so strongly to make whey protein palatable that you won't even notice.
Stop substituting and finding short cuts. Just eat a balanced meal with vegetables.
I wouldn't sweat it - just focus on eating unprocessed food fiber or not
A single bowl of oatmeal with chia seeds gets you about halfway there. 2-3 tbsp of chia seeds has ~10-15g of fiber, mostly soluble fiber. Soluble is what you want for lowering cholesterol because it binds to bile acid, which means your liver has to use cholesterol from your body to make more bile. The oatmeal adds another ~4-5g and is largely soluble too.
After that you don't have far to go. Besides lentils and many beans, whole grains are really fiber dense, e.g., having wheat bread on a sandwich can get you another +6 just from the bread. Brown rice instead of white gets you 3-4 more. Weird grains like barley and bulgur have a bunch and go well in things like casseroles, chilis, etc. You'll also get at least some from whatever fruits/veggies you were going to eat anyway.
It is a lot compared to what most people take, but it’s not hard. Banana, orange, some broccoli, and you are close. Lentils, beans (lupini is great because it also has protein) here and there, oats, an extra fruit (pear is nice), berries, avocado, so many options.
But you need to be conscious. Most people need less starch and sugar and fat.
I poop 2-4 times a day. I know some people have a real problem going to the bathroom once a day… just eat more fiber folks.