Have mild acne and post inflammatory pigmentation
If your skin is tolerating differin and the rest of your routine well, adding a product with azelaic acid can help with the pigmentation (as well as acne). Also, this ingredient is effective for reducing dark marks: https://whatsinmyjar.com/skincare-ingredients/isobutylamido-thiazolyl-resorcinol. This active is patented by Beiersdorf (Eucerin, Nivea brands). Unfortunately, most of their formulations with it contain fragrance. This serum though might work for you: https://whatsinmyjar.com/product/eucerin-anti-pigment-spot-corrector. It has a high content of drying alcohol, so it is important to apply it precisely to the pigmentation spots and avoiding the rest of the skin not to dry it out. It's also important to apply it only on fully healed spots - it can interfere with healing of the fresh ones.