What is a good justification to extend my DEROS? Is it possible to match my DEROS with my ETS date?

Before I retired, the usual stance of the AF was if your DEROS and your ETS were within 180 days of each other, you were automatically extended in place until your ETS. The justification was they didn't want to pay to move you twice within the same fiscal year. I would assume under JTR rules/policy the Army would be similar.

Do you plan on staying in Hawaii? If so, that could be your justification.