Hello, wanna see if I can give this a try before giving up and taking it to a shop.

I bought a 86 300zx a few months ago for pretty cheap due to car wouldn't start and owner gave up. So I did a full tune up replacing plugs wires cap, tps sensors, checking relays, fuses, pressure on fuel pump, new battery, ECT ECT. Car still wouldn't start, fast forward several infuriating hours later turns out it was the ECU had a small burn out on only the fuel pump continuous run, so it'd prime but shut off after it started. Cool, did a shunt on that cause don't got a grand to rebuild an ECU just yet.

Drive it around for a few days and notice head lights dim out, get alternator tested, failed, replace alternator. Worked for a couple days then dead battery, tester says alternator fail. I tested it before I put it in, so figured original wiring from alt to battery is bad, replaced it. Works, drive it everyday and starts up perfect. Go out of town for the weekend and come back, dead battery. Took it to a buddys shop to get water pump replaced and asked him to check out the battery and charging system. He charged battery, said it didn't have a draw on it, and said alternator was working. He keeps an eye on it for a few days and starts up every day, but when I go to get it he didn't start it the day before, and once again dead battery. He had no idea, and I'm running out of them too. Also did the dummy checks and disconnected radio, no after Market security system, door lights shut off, ashtray light is removed.
Any help or ideas is much appreciated

Welcome!
You have posted this in the Z32 section, your best bet for help would be to post in the Z31 section for best results as we here are in most cases unaware of Z31 repairs and procedures.
Good Luck!

Moved!

Also one surefire way on a car this old with this many electrical gremlins to make sure the battery won't drain is to install a killswitch.

You replaced the alternator, there is now drain on the system, so.... bad battery?

Pull the fuse for the stock alarm. I had to do that a milion years ago and it was a drain for sure. These cars stil seem to drain at the end of the day but this will help