Hi and welcome. Your form is ok for approach shots for the distances you mentioned. With a harder acceleration of all body parts a controllable flat land 350' is very achievable. That would translate to around 400' golfable low line throws. If you want more distance check out the right pec drill thread. Basically you want the elbow to get closer to the target before straightening the elbow. If you are less flat footed you gain power slightly and won't hurt yourself. Then it is safe to plant x step and plant step pointed farther back up to 180 degrees away from the target. So that you'll reach back and accelerate longer and get the disc out faster. Pulling the disc close to the chest will lead to a tighter bend in the elbow tendons, which stores some of the energy of the forward motion into the tendons, which releases as an atypically fast elbow chop, which increases speed and spin on the disc=distance increase on straighter lines with less turning over and fade. Swinging the left arm closer to the body and farther forward at the disc rip from the fingers gives a little more D.
Flat shots need running on the center line of the tee and planting each step on the center line. Anhyzer needs running from rear right to front left with the plant step hitting the ground to the left of the line you're running on. Hyzer is the mirror of that.