Have been reading lots of threads trying to determine whether the ONLY way to program a G key (2018 TRDORP) is through Techstream... or if there is also one of those long procedures of opening & closing things & turning keys that will also program a G key?

From various posts, clearly Techstream will work. But I don't have Techstream and don't own a Windows computer.

I still have one functioning G key, but just lost the other one. (in the past, that mattered, but I'm wondering now if it no longer matters whether you still have a functioning master key?)

Can anyone state DEFINITELY whether there exists one of those "stand on your head while opening the door three times in a row" procedures for 2018's??

Ace Hardware programmed my keys that I bought on eBay for $25.00@
Only if they don't cut the key. I have a key cutter.

Nope. My neighbor has a 16 Premium and we were able to cut a key on my machine and then use my Autel MK808 to program it to work. Should work as long as you don't have a smart key setup (Never tried it on a smart key version)

A few years ago I had a fiasco with my 2018 TRDOR keys. I was trying to add a key, but the programming kept failing. At first they thought it was a bad replacement key, but later they figured out that my vehicle already had 6 keys registered so they could not add the new one. They had to first clear the memory and then add them all back one by one (I had to have all keys present). Anyway, between lost keys (they stay registered), valet keys, spare clicker keys, and hide-a-keys you may run into this problem in case you weren't aware of the limit. I gather some cars have 8 slots, but not mine.