Can I visit Thailand again under the visa exemption scheme if I've already done so twice this year?

not by air

It depends on how you entered. Such short stays are not a problem and you can enter without worry about being questioned about what your true intentions are (you're clearly not living in Thailand). But there is a limit on the number of times you can enter by land: that's maximum two per calendar year. So if the two entries you refer to were both in 2024 and both by land, then indeed you need a tourist visa now to be able to come. However, flying would also work.

It's only 2 times per year at a land border. There's no written regulation on how many times you can enter visa exempt when you're flying in. You should be fine.