Hugs to you for what you have been through. I have had three losses and the third was so hard that I knew I never wanted to go through it again.
Everyone's decision of when they are done and not done are their own personal decisions. Some people would probably judge me for going too far and some for not going far enough in assisted reproduction.
I will answer your question about how I knew we were done.
I had a DS with a picture perfect pregnancy aside from maybe gaining a few too many pounds and a perfect delivery and very healthy adorable kid. We didn't struggle to have him, but I did have endometrial scar tissue removed as I had complete blockage of both tubes and adhesions on bowel, bladder, uterus, tubes, and ovaries. Once we got pregnant and delivered the doctor assured us that we wouldn't have issues having another child. I nursed until he weaned and then started trying for number 2 when he was 2 years old. We struggled and took clomid and tried other interventions until finally we were convinced to do IVF as we were "perfect candidates". Healthy, no reason for infertility, healthy previous pregnancy and we were only around 30 years old and had great AMH and great sperm count, though slightly low motility. My first IVF resulted in twins that I lost. I then had a saline ultrasound later that year and got pregnant on our own that cycle and lost that one at 5 weeks. We started another cycle (my DH was probably done at that point but I wanted to give it one more shot). That cycle I was overdosed on a med due to pharmacy error and cancelled cycle several days into stimming. We waited and started again right before Xmas. I again got pregnant and again lost the baby. I then spent over $1000 on genetic testing and to see a miscarriage specialist only to not be given any answers. We still had, I think 12 embryos left from that IVF but we decided not to pursue a transfer again and donated them.
My DH and I made a deal before the second IVF that we would pursue both options at the same time, foster care/adoption and pregnancy knowing that we wanted to adopt eventually anyway no matter the outcome. We were going to do foster care adoption so we had assumed it would be likely to get a 2-5 year old so if we had gotten pregnant we still would have done foster care. A year after my third loss we had a beautiful baby girl placed with us through foster care that we adopted. Another year later another beautiful baby girl that was placed with us that we are adopting.
My husband and I always wanted to adopt, so it was a decision we would have made eventually anyway. I, like you, have an adopted sister so it pulled on my heart and I knew that bio/adopted is no difference in love.
Good luck in your decision and your path forward. No one can tell you when it is the right time for you to step away but hopefully with others stories and experiences you can settle on YOUR best decision.