What happens if I arrive in Thailand on the last day of my 6-month METV?

Yes, that's exactly how you can get the most days out of the METV. If you don't do that, it's essentially a double-entry. Can also extend for 30 days once back.

Try not to come back on the last day/s, since your transportation could easily break down, your plane could be cancelled, or many other things could happen, preventing you from re-entering that day.

21 March 2023 is the last day you can enter Thailand using your METV to get 60 days stamp. The answer is yes, you will be stamped for 60 days again when you enter Thailand on 20 March. That entry stamp is eligible for 30 days extension for tourism purposes.

So the last entry stamp of 60 days + 30 days brings your stay until almost 9 months (270 days) after your visa is issued.