Look up the nordic, and reverse nordic curls, and tibialis raises and TKE exercises. There is a youtuber who has popularized them (kneesovertoesguy). I've found they help me immensely when it comes to stiffness/soreness in my knee (had knee surgery a year ago). If I was better about it I would be more regular about doing these exercises regularly, but alas I am too lazy. (I also have flat foot issue and found "racing flats" with not too thick soles to be my ideal, even running marathons).
