What are the visa on arrival restrictions for US citizens returning to Thailand?

You should not have any problem, but they will put a "Visa Exempt" stamp in your passport rather than a visa on arrival which is not provided for US citizens.

First off let's get your terminology back on track

There is no 30 day visa on arrival for US passport holders 😮

A visa on arrival is SOLD to people from 17 countries for 2000baht when they land here and is good for 15 days

you're talking about a free 30 day visa exempt entry stamp (that people from 52 countries get when they show up here without having bought a visa before hand).

Now whether you will have any issues or not is totally dependent on the immigration officer you get at passport control when you stamp in. Could you have issues and be questioned? You could but I'd say it's rare. Have your onward flight showing when you're leaving the country and I doubt they'll say anything at all when you stamp in

OR

You could certainly get a single entry tourist visa online with the thai consulate eVisa website here. I don't know it'd make a difference either way, BUT this is where you apply