1. What format are you using? National Dex Porygon-Z behaves much differently than base game Porygon-Z. I'm going to assume Gen 9 formats for now
2. Analytic is strong on slow and tanky Pokemon that can afford to take a hit OR Pokemon that threaten a lot of switches, like Starmie. In an OU context, Porygon-Z, while strong, doesn't threaten a lot of Pokemon out with base 90 Speed, and its bulk also isn't the best. Adaptability lets Porygon fire off base 160 power Tri Attacks after STAB.
If you were to use Porygon-Z in the lowest tier possible (NU), Download is an actually viable option, since there are a lot of Pokemon that allows you to grab a SpA boost (Overqwil, Registeel, and Avalugg, for example) I'm don't play NU, but someone like RisingManectric should be able to help you if you're trying to use Porygon-Z in NU.
3. As for the Thunder Wave/Nasty Plot moveslot, Nasty Plot works, but Thunder Wave doesn't really work that well. This is because other Pokemon, like Galvantula and Kilowattrel, work much better as paralysis spreaders than Porygon-Z. By contrast, Pory wants all the coverage it can get to hit as many Pokemon as it can, so a basic set would run Tri Attack and 3 coverage moves.
4. For the EV spread, you should just fully invest in Speed, and Timid is usually the best nature. Like I said, base 90 isn't really fast, so you would like to run as much speed as possible with your EVs and nature.
5. For the item, a basic set would run Choice Specs or Choice Scarf. While Adaptability Tri Attack is strong, you can't hit anything for Super Effective damage (hence why Download is viable). Specs can overcome Tri Attack's lack of strength with pure power, while Scarf allows Porygon to outspeed faster but frailer Pokemon, like Infernape and Mienshao. This does mean dropping Nasty Plot for a coverage move, though, so if you want to make a Nasty Plot set (which isn't recommended due to Pory's low speed), you can try running Life Orb.