In used to use bus tickets to get around this option. $10 Hat Yai to Penang. Demonstrates onward travel and doesn't cost very much. However, I haven't done this recently, due to covid so I am not sure this is an option.
Provided you had all the documents in order, it would take an hour from the time you land until you are outside the airport. BKK Airport has a train that can take you to the heart of the city, its about 1 hour ride. You're looking at minimum 4 hours to/from maybe more. That gives about 6 or fewer hours to mess around in BKK. To play it safe you would want to keep it to about 4 hours of messing around downtown.
I guess its worth it, but it's kinda of a pain in the ass. If I had never been here and had no plans on returning, I would do it.
I thought I read the PCR test had to be in English. I don't know about the cert. I was in Mexico before coming to Thailand in February. I am not sure if you can just print out an English version of your vax card, replacing all the pertinent information, government clinic id, vax batch number, or whatever they put on the cert.
Consider applying for a visa in the US. As you pointed out, it's all online now. If you don't need to send any documents you may be able to get your visa.
For US citizen (the only rules I am familiar with), its 60 days + 30 days extension. However, people or opting to go on Covid extensions which seem to be indefinite
I believe you can get a exit permit. Meaning getting the first visa 60 days, depart to Vietnam, getting exit permit on your way out, and you should be able to return without issue.
Also, I think you will be fine getting two visa exempt, especially if you dont have extensive history in