Lol I also have flat feet. It's not an issue, you need custom-made insoles for your shoes, and I have two pairs: one for my sports shoes, and one for my every shoes. They're the exact same insoles, it's just that I was tired of changing them out every time (also helps their longevity) so I got two made.
Resistance training is great for mobility, the stereotype of the huge buff dude who can't scratch his back is just a stereotype, muscles are the basis of mobility, and training them with resistance improves their elasticity as this is what they are, springs ๐
Thank you comrade, that was the whole reason I set out to writing this guide in the first place.