implantation dysfunction

Listened to you on a podcast recently and appreciate you so much for sharing your knowledge on this topic. Wondering if you have any input for next steps after 2 implantation failures (of 5AA embryos without PGT-A testing, female age 37, male age 45; unexplained infertility after previously conceiving 3 years ago without issue. C-section at full-term due to fetal heart decelerations. Tried naturally for a little over 1 year. No positives). AMH 1.7, 7 untested embryos combination of, 5AA, 5AB, 5BA, 6AA. Lining at modified natural trigger days 6.7mm and 7.5 mm. Only taking letrozole days 3-7. Cycles short and typically ovulate on CD10-11. Based on what you said I suspect lining suboptimal but worried about any consequences of c-section and uterine environment being hostile to embryos. Wondering if there’s anything I should switch or investigate before using any more embryos or if at this point you would suspect lining being the next likely culprit.