I get it, I also call it just Thailand. But it seems especially the expats here love the Kingdom thing 😂 (which is not wrong, technically speaking). Many people actually just call it Thai (to refer both to the country or its people). In my opinion, the shorter the better.
You're not getting a visa on arrival, those can't be extended (only 15 days and that's it). You're entering on a visa exemption. Diferent terms, different things.