What should I know about the 60-day entry limit with the METV visa for Thailand?

Under ideal circumstances a METV can last almost 6 months.

Near the end of your first 60 days, extend the visa at an immigration office, which adds 30 more days (for a total of 90 days). At the end of those 90 days, leave Thailand briefly. When you return, the immigration officer will give you another 60 days. Near the end of that 60 days, go to immigration and get a 30 day extension. That's a total of 180 days.

The only problem I found is that, the METV visa has a 6 month Maximum expiration date. Immigration won't give an extension if it finishes after the Maximum expiration date.

Keep in mind, the main purpose of the METV is make it easy for tourists to go in and out of Thailand while on a several month trip to SE Asia. Although it can be used to stay for almost 6 months, that's not why Thailand offers it.

Also, AFAIK Thailand will not issue more than one METV to a foreigner in a 1 year period.

If you want to stay longer, get an education, retirement, or Elite visa.

If metvs were 6 month stays, every foreigner in Thailand would be using them to live here. See above

You can extend the stamp by 30 days at immigration. Then you can leave/re-enter for another 60+30, and if there's still time left on your visa, bounce again for another 60+30. You can get almost nine months

That’s how the METV works. It’s valid for 6 months from issue. Every time you enter Thailand you are stamped in for 60 days. You have the option of extending that entry once per entry at a local immigration office for 30 days (1,900 baht) or do a border bounce and re-enter Thailand for another 60 days.

Do a bounce just before the visa expires and you’ll get another 60 (plus 30) days. Depending when you applied you can get almost 9 months with that visa.