What else can I try for scar hyperpigmentation from tummy tuck several years ago? (photos)

It generally takes about a year to a year and a half for a scar to fully mature.  Since your procedure was done several years ago, non-surgical modalities such as scar creams, lasers, microneedling, etc., may not work anymore.  Re-cutting and re-suturing the scars might be your best bet followed by use of those other non-surgical things on the fresher scar.

Thank you for sharing your question.  Improvement can be had with a number of different options and the key is to identify your major area of concern.  If only the hyperpigmentation I would consider a straight lightening cream because although Triluma did not work the retinol in it was possibly the cause of your irritation. There are also other laser treatments and chemical peels that can be performed to help with PIH.  If the thickness of the scar is causing some of your concern then a scar revision would be a possibility.   

My suggestion would be the usage of Laser.  Broad Band light Laser with a combination of profractional, has been used several times on our patients with wonderful results.  

It appears from your photos that your scars have not totally matured.  Scar revision may be a good option for you.  At this stage, the tension of the skin will be less and revision could give you a much better result.  Discuss this option with your plastic surgeon.