When and how should I apply for an extension for my multiple entry tourist visa in Thailand?

With METV (and probably all visas) you don't actually extend the visa, you extend the stamp (permission to stay). So if you stamp-in for 60 days, you can extend that stamp by 30 days, given you 60+30 days per stamp/entry. After 90 days you must leave the country and stamp back in for another 60 days, then repeat. You can get close to 9 months on an METV. In theory you can have a full 9 months but depending when you activated the visa (first stamp) and when you finally leave you may loose a few days.

To avoid the Pattaya Immigration office I am choosing to not extend my visa stamp, but rather I will do a border run. In my case I will stamp in for 60 days, at/about day 60 I will travel to Cambodia, have lunch, come back. I may extend my last stamp or just use my METV for 8 months.

Are you aware you can get 8 to 9 months out of the METV visa?? Check the expiry date on the visa, it should be valid for 6 months, if you do a border bounce just before it expires, and after that a final 30 day extension, that gives you 8 to 9 months in total.

Go to the immigration office you will be doing the extension at, and get a list of requirements, for instance some will require a TM30, some won't, Just do the extension when you have 7 days or less left on your current stamp, because some immigration offices won't allow you to do it earlier than that, the 30 days will be added onto your current stamp end date.

Most offices allow you to apply for an extension 30 days prior to your stamp expiring. Jomtien, Samui and Koh Phangan will want less than 7 days left.

They add on the days to your existing stamp so no downside to going early.