The last time I left the country was January 2020...so I still have the same TM6 card from that last entry back into Thailand. It's just further evidence of your last entry. Thailand is the land of paperwork (as a kid in school, we called it busy-work)...there's a lot that doesn't make sense here, and Thai bureaucracy will drive you nuts if you let it.
I order from ebay all the time...shipping can be expensive sometimes, but it works for me. Also, Aliexpress is good, but it's most often cheaper Chinese goods...
The key is to go to the district office where you will actually sign the paperwork and get married, then talk to them beforehand about what documents you need. Not every office wants the same docs, so ask them what they need, and then give them exactly those documents. My local office wanted one of the witnesses to have the same last name as my wife, but this is not something all offices will ask.
I did it in Bangkok during 2020. Go to your local office and ask them. They're the ones who will approve or deny it; you need to follow THEIR rules, not some national standard, as you might expect in many other countries.
There are still millions of land mines all over Cambodia, so you can go exploring in the cities and in certain areas, but if you want to go camping or hiking or get away from the cities, that'll be an issue.