It's hard to tell as the only video that shows it is the first but it looks like you're only pivoting on your toes which takes a lot of power out of your throw and really wears on your knee over time.
here is a link to a great clinic that taught me how to pivot on my heel as opposed to pivoting on the toe.
You're also beginning your pull through before you've finished planting with your lead foot so you're also losing distance because of bad timing. Slow your steps down.
I'd be willing to bet that you're also strong-arming your discs as well.
How far can you throw a putter? I only ask because it takes a lot better form to throw a putter 300ft than it takes to throw a driver 400.
Do you pinch your disc before your release or are you death gripping it the whole way through?
Does your arm get tired throughout a round? If so you're pulling through too hard and you need to relax your arm and work on pulling through smoothly. Think about whipping a towel; it's not about the forward momentum you start the towel with, it's about the lightning fast acceleration at the last second. Throwing a disc is the exact same.
The other guys had some great advice too. You ought to disc down some and only stick with the katana and maybe go to a blizz boss and then get some slower discs. Really anything on a universal flight guide that is 11speed or lower. There is no shame in discing down and it's not a permanent change but it will teach you better form and you'll see better performance out of your discs.
edit: I'm 6'1, 205, avid weightlifter, and max out at 500'(not very often though)