"I plan on spending 4-6 months a year in Thailand" really??? i did exactly that with multiple entry. guess from now on, i'll just read here. how one legally stays in Thailand for 6 months a year, without it, leaving that entirely up to you. also in US good luck finding a doctor that will state you don't have elephantiasis and all the other crap on the medical form, with all the liability they're subject too, the background check every year.... lot more work that I wanna do... but hey you do you, and since its your channel, you can beat up on anyone you want, that has a different opinion.
yes that's exactly what it means. As others have said the us considers all citizens residents and pay any taxes due, and states follow suit. The best strategy if you live in high tax states, is to move long enough to claim residence in a no tax state. Then move to Thailand. However many high tax states can be very aggressive in challenging your move,.so best to read up on the subject and do it carefully and thoughtly. I.e. move bank accounts out of the high tax state. I read one article that some state was successful solely based on a pet, maybe left with a relative the article didn't explain the exact situation. Drivers license, new mail address, bank accounts, anything that can tie.you to that state. If you have very little income probably not worth their time.
If he's German, and in Germany seems the simplest thing would be to do a German Will and/or Trust. AFIK none of these require any marriage certs, etc to be written.