What should I do if I want to leave Thailand at the end of my Non-B visa?

Follow the rules and no problems.

Nothing happens, they'll stamp you out without any issues.

but with that being said

if/when you come back, should you ever try to go for a long-term visa/extension they will see that you didn't follow the rules like you were supposed to and cancel your previous extension based on employment before you left and they will fine you.

That's because just stamping out of the country does not cancel an extension officially with the immigration office it just lets you leave.

Now on the rare chance you are on the initial 90 day entry from a non-immigrant type B Visa based on employment (as in, you have not ever extended it), you can just leave in that stamp will cancel when you go out