I have IBS-D. Generally, I consume over 40 grams of assorted fibers each day. I cannot tolerate supplements. I generally average around 48-50g of fiber (around 50-ish, lately, because I'm back to my vegan wits).
Are you consuming enough fluid throughout the day? I find that steady doses of fiber work. I do think you might want to do a little bit of an elimination trial with some of your frequent foods (i.e. oatbran, corn chex) to see which one might be the culprit.
I like your idea of strawberries! Do you enjoy smoothies? Do you have a food processor? It makes IF (generally, i.e. when stable) a breeze--in combination with SF, of course.
I understand your frustration with Seroquel and it's constipating effect. I am sorry.