Is it necessary to buy a 30-day visa extension on a METV after the initial 60 days, or is it simpler to leave the country each time?

Entirely depends on you. I did the extensions on METV, as it’s cheaper and more convenient than border bounce. And I only had to bounce once instead of 2 bounces for my stay.

Less than total 4 months, I guess it’s the same as you need one bounce anyway. If 4+ and up to 9 months I’d definitely do extensions to save money and trips.

If you want the maximum time out of your METV almost 9 months you’ll need the extensions your re entry is built into your visa

No, no need to apply for 30 days tourism extension at all unless you want to. It's your choice and decision if you do border bounce at the end of the entry stamp of 60 days or apply for an extension. All people have their own priorities and criteria. Also depends on where in Thailand you would be at the end of each entry stamp.

As long as your METV is valid, you don't need to get a re-entry permit to re-enter on that visa. Multiple Entry is embedded on that visa.

you may want to check this post about METV as well.

1/