How can I treat these malar mounds/festoons that I developed after getting under eye filler (Restylane Lyft). Need help! (Photo)

It does not surprise me you have festoons after 6 treatments with hyaluronic acid fillers for your lower lids...since the mechanism of action of the hyaluronic acid fillers is that of a sponge....they take on water and swell.  Then they get resorbed.  This back-and-forth cycle of swelling and deflation can actually reduce the tight coiling of healthy collagen/elastin.  So repeated injections of those fillers in the same area can lead to laxity in the skin.
I recommend having a biostimulatory agent (Sculptra, BellaFill, or Radiesse) injected into the lower lid area, as it will trigger 2 kinds of collagen synthesis, including a stiffer one to fill in the hollow, and a softer one to improve skin texture, including improved dermis thickness.  Additionally, a skin-tightening procedure done about 2 weeks to 2 months over the top of the (biostimulatory agent treated) area will augment results. Skin-tightening procedures would include things like microneedling, microneedling with radiofrequency (such as Pixel8-RF or Morpheus8), a medium-depth chemical peel, Vampire facial (microneedling with Platelet Rich Plasma), Ultherapy, and certain lasers.  Another recommendation it to take Vitamin C  (at least 1000mg daily) for about 4 months after the injection to optimize collagen synthesis (since vitamin C is a co-factor for collagen synthesis).  Yet other things that can increase the tightness of skin include Celluma medical-grade LED light therapy, and using medical-grade skin care (retinoids, human stem cell growth factors, and micro-technology products).

Eyelid skin is 1/7th as thick as cheek skin.Amuch thinner filler such as Silk or Volbella is better to use. The proper dose for hyaluronidase is somewhere around 30 U for each 1/4 cc of filler used.Juvederm Ultra requires a little more in my opinion