If you time it right, you will be able to stay for 9 months with your METV. Make sure you last trip arrives in Thailand before the "enter before" date on the visa to get another 2-month stamp, and extend that for a further 30 days. You are only allowed one METV in any 12-month period.
A senior official in Chiang May is on record as saying it is fine for digital nomads to work on a tourist visa as long as customers and payment is all overseas. Staying long term on a tourist visa is not legal, however.
Expiry dates are fixed. You can go out and come back in within the date if you have a reentry permit. You can also extend the expiry date with the appropriate fee. But you cannot alter it just because you are not here.