#1 - No reason to double tint the new Tesla glass, it already has tons of heat & UV rejection.
#2 - Yes, ceramic films are worth it because they are functional (reject heat/UV/IR) vs just looking dark for privacy. Darkness is up to you, but here's a good video comparing day/night %.
#3 - You do NOT want the ATR series (Metallized Window Tint) this can cause issues with cellular signals, vehicle communications (WiFi) and radar detectors if you use one.
#4 - I just did my Ultra Red X fronts in 20% ceramic to match the rears for $200. Between 15-20% you wont really tell the difference, whatever shade is available in your film type. For the windshield I went with XPEL XR Plus 70% for $650. These are VERY expensive to do in a full 1-piece. Most places in the NJ/PA quoted $550-1200 in ceramic film.
IMO I wouldn't go any darker than about 50% on the windshield and 15-20% on the front side glass. I had to turn off the Auto-dimming mirror option because with the 20% on mine I couldn't see anything on the side mirror when they dim at night, but its fine with the tint alone. I would not put any more tint on the rear glass unless you're an owl if you want to see anything at night.
Note: LLumar Auto Film spec sheet attached. CTX, IRX or AIR series films are the only ones I would put on my vehicles.