Dent/ dimpling after thread lift (over 1 year post op) - how can this be fixed? (Photos)

I stopped performing threadlifts quite a few years ago, following reports of an unusually high rate of adverse events related to their use, such as infection, scarring, "spitting out" of the threads, and unwanted dents and dimples, as in your case here.
Regarding the lack of success with subcision, you did not mention how many treatment sessions you had. it should be emphasized that optimal improvement from subcision typically require a series of treatment sessions (on average about three or four sessions).
Another way of addressing the dents and dimples would be the use of an appropriate filler properly injected by an experienced, board certified cosmetic dermatologist injection expert . Generally speaking, the results of fillers are immediate, and the treatments typically engender little to no significant downtime. Hope this helps and best of luck.