...When I asked them about how come it is difficult for my tesla to reach 300 miles of range, they simply ignored the question and stated they did a BMS check and nothing was wrong and that if I drove slower, I can achieve it.
There’s nothing “ wrong” with your car.
Your car would never do 300 miles of 65+ mph freeway driving on a single charge - not when it was new and certainly not now. As has been stated as nauseum, the EPA’s range test’s average speed is 48mph. If you average faster than that, you won’t be able to make the EPA-rated range.
Some range loss is normal as the battery ages. My guess is that you haven’t lost as much range as you seem to think you have.
How has your “range loss“ affected you in your day to day driving? Are you not able to make it from Supercharger to Supercharger on your road trips? I understand that you might have to spend 5 minutes longer at a charging stop, but how else have you been affected?