Congratulations!! Welcome to the brotherhood.
As far as your questions go -
1) why is the stereo so terrible?
It's a cheap sports car. Sports cars that sell new for under $30k (usually under $25k) generally don't come with fine sound systems.
As you get older and your hearing degenerates, the sound system becomes less important. I used to be a hard-core audiophile. Used to have a deeply wrong but very passionate relationship with a Marantz tube amplifier. I could tell the difference between analog and digital music within 2 seconds. Now, at 60-something, I'm lucky if I hear anything.
Roll down the windows and listen to the sound of the wind rushing by. It's better than most things on the radio.
2) why are the gauges so hard to see during the day time?
As @
DarkPira7e suggested, adjust the illumination. Should be fine if turned up.
3) why is there little thigh support in the lower seat cushion? 800 miles hurt my hamstrings.
I don't think the car was really designed or intended for 800 mile trips. This is a cheap sports car for relatively short blasts down twisty country roads. I find that my aging 60-something body is fine in the BRZ for up to about 3, maybe 4 hours at a stretch. If the trip is longer than that, best to take a different kind of car.
Have fun!! The days of pure and inexpensive sports cars are waning. Enjoy it!!