Nope, cannot get Supplemental Security Income (SSI) in any foreign country (outside the USA), but getting Retirement Social Security (RSDI) anywhere in the world as an U.S. citizen is no problem
I was just about to comment on being some of the first to cross the new Friendship Bridge to be opened at Beung Kan–Paksan. As a side note, I have also crossed many of the Lao–Thai borders and find them quite fine and generally easy to cross....
I am over-simplifying the particular requirements and details of the DTV Visa, but think about DTV Visa like it's a 5 Year Multiple Entry Tourist Visa where they are 2 main pathways to qualify for the DTV Visa:
1.) Remote Work, Digital Nomad, Workcation: Work done for things outside Thailand and income is mainly derived from outside Thailand, but you're doing such work virtually while physically inside Thailand;
2.) Thai "soft power"/medical tourism/long term medical visits: "Soft Power" can emcompass many things, but so far, have seen Muay Thai training courses, some have been able to have long-term Thai cooking/Thai cuisine courses, medical visits encompassing long-term care and several visits over a prolonged period time....
Having said all this, perhaps looking into a Thai marriage visa or Thai visit family visa might be a better option for you if you are not going into those 2 main pathways for the DTV Visa....
China has multiple arrangements from Bilateral Visa Exemption (ผผ. 30), Unilateral Visa Exemption (ผ. 60), and even grandfathered Visa on Arrival (VoA), but the simple answer to your question is 60 Days