I too had flat fleet, and wore arch supports for many years, but after training in barefoot running for a year I "fixed" the issue. I even ran a marathon in barefoot shoes! It sounds like an advertisement, but it was life-changing for me. (I'm also a big guy at 240lbs)
The book I used is "Chi Running" but there's many similar books.
Yeah, not quite the same but my arches stabilised by moving to a more forward strike - mid to forefoot. We've been careful with my son who is now 6 to have flexible soled shoes and he naturally runs like this (may not be related, but interesting to note).
Glad you asked here. I was taking down notes during the article to inquire about if this would help me get into running. I have very flat arches as well, but I've never been a runner and my knees are already pretty bad.
The book I used is "Chi Running" but there's many similar books.