Which surgical procedure would address my sagging lower butt? (photos)

Thank you for posting your question here!My Answer will be based on the limited information and pictures provided, for a detailed and bespoke plan please consult with a plastic surgeon in person.
From the images provided , you do seem to have some skin laxity and sagging , this can be addressed with a surgical buttock lift , which when couple with autoaugmentation can give you god results . Other options can include a thong lift, or implant for projection with a lower buttock tuck. I would recommend that you consult with your plastic surgeon in person so they can examine you and come up with a personalized plan that works for you.
I hope this helps, best of luck!

In regards to treatments that going to have a big impact or make you look much better I would say they’re really aren’t any.
You have a subcutaneous fat distribution that has very little fat on your lower extremities and the transition from your buttocks to your thighs is dependent on sufficient volume to look good.
Fat grafting would be the treatment of choice but in order for this to work, there has to be a sufficient layer of tissue to graft into.
In other words, the people who need the procedure the most end up not being good candidates for it.
I’m afraid only very thin layers of fat would survive fat grafting in your case and for this reason only very small improvements are possible and it’s a pretty involved procedure so in the nutshell, I think my recommendation would be to not move forward with surgery.
There may be plenty of plastic surgeons who are far more optimistic.
Recognize that this is how we make a living.
Before, even contemplating having any surgical intervention, you should ask the providers to show you as sufficient collection of before, and after pictures of previous patients who have very similar body characteristics to your own.
You need to bring pictures of yourself to use as reference.
Close attention to how thick the subcutaneous fat layer is on the example pictures.
I don’t think you’re going to see any long-term quality results on people who had the same characteristics.
Finally recognize that early fat transfer results Can look very impressive but early results do not represent long-term final results.
In order for after pictures to be representative for fat grafting surgery they need to be at least 3 to 6 months from the date of surgery.
Best,
Mats Hagstrom MD