Routine help: when to use barrier serum?

I started using tret 0.025% about three weeks ago. Currently using every 3 days, planning to go up to every other day next month. My dermatologist also prescribed clindamycin, and I'm supposed to use BP wash with it to prevent antibiotic resistance. She also prescribed 15% azelaic acid, but the prescription never got filled because of some insurance weirdness. I already had 20% azelaic acid (Melazepam) on hand, so I've just been using that instead. Current routine: AM \- 4% benzoyl peroxide wash \- clindamycin lotion \- moisturizer (either Dr. Althea 345 Relief Cream mist or Neutrogena Ultra Gentle daily facial moisturizer) \- Elta MD UV Clear SPF 46 PM (tret night) \- Vanicream gentle facial cleanser (sometimes makeup remover beforehand) \- tretinoin 0.025% once skin is dry \- maybe moisturizer (Neutrogena Ultra Gentle or Vanicream in the tub), but I've been experimenting with just doing the tret by itself, per some posts on here PM (other nights) I've been alternating 1) tret, 2) night off, just moisturizer, and 3) azelaic acid + moisturizer -- planning to start alternating tret and azelaic acid in a couple weeks. Have had a little peeling, but not all the time. Acne seems about the same, can't tell if I'm experiencing any kind of purge yet. I just got a barrier repair serum from Cocokind -- what do you think is the best time to incorporate that? Thanks!