@Rob *****
Assuming travel gets back to normal my suggestion would be ONLY use the Thai passport for ASEAN based travel, in particular land based travel. You can not switch passports at a land border.
Unless intending on spending a long time in a country for which US citizens get more time than Thais (Singapore, Malaysia and Brunei I think) then using a Thai passport is better. Again, for overland travel to Malaysia and Singapore the Thai passport must be used.
The Thai passport (or rather, ID card) will be useful at local border crossings where third country foreigners aren't allowed to cross, or at some border crossings where foreigners need to pay for a stamp and pass through formal immigration. For instance, if going on a day trip or for up to 1-2 weeks to border areas of Myanmar, it is much easier, cheaper and faster to use a Thai border pass than a passport.
A foreigner going to Myawaddy across from Mae Sot on a day trip needs to spend US$10 or 500 Baht, go through immigration and stand in line with all the other foreigners, walk or drive across the bridge and then spend a long time lining up on the way back. The whole process can take 2-3 hours during peak periods, though an hour all in all is the average.
A Thai with a border pass can not only spend up to 7 days in Myawaddy, but it only costs 30 Baht on the Thai side, with another 20 I think it is, collected on the Burmese side. There are also separate windows for Thai border pass holders allowing much quicker access to the country.
Kanchanaburi allows Thais to travel to Dawei and it's beaches for 1-2 weeks for just 30 Baht plus another 30 on the Burmese side with just a border pass. Obviously they must head back the same way they came, but for a quick beach getaway where further travel is not desired this is much easier and cheaper than having to apply for a visa at the embassy in Bangkok or consulate-general in Chiang Mai (e-visas aren't accepted at this border).
Borders closed to foreigners such as the Three Pagodas Pass and the Singkorn pass allow Thais to cross into the country using a border pass. Singkorn allows travel up to Myeik on the coast.
Ditto for all the other local border crossings between Thailand and Myanmar, Laos, Cambodia and Malaysia that are not open to non-locals.