Which procedure would address my sagging lower face and under eyes? 31 y/o (photo)

Hello and thank you for your question. There is only one photo to review which does make the assessment more challenging. You may benefit from injectable filler to the lower eyelid/cheek as it does appear as though you have developed a malar groove. You could also consider laser treatments to this area. A detailed assessment by a board certified facial plastic surgeon may be of great benefit to you.

First off, you would be wise to exercise a great deal of "buyer beware" caution if offered treatments with expensive, light or energy-based treatments as the hard science to support the use of these gizmos lags well behind the heavy marketing hype and social media buzz that typically surrounds and promotes their use.
Secondly, we have known for years now that early intervention, I.e. "prejuvenation", using Botox, fillers and peels, for example, coupled with the at-home use of Rx topicals, can delay or obviate the need going forward for undergoing expensive, aggressive, and invasive surgical rejuvenation procedures, like facelifts and necklifts.
On a personal note, I started my daughters-in-law with Botox and filler treatments in their very early twenties. And now well into their mid-forties, they still do not require any surgical rejuvenation procedures.
You would be wise to consult with a board certified cosmetic dermatologist injector expert who can select the appropriate fillers to address your areas of concern. Hope this helps and best of luck.