Unfortunately, I was in the same situation two years ago.
We tried to have a child for two years before we became pregnant (early to mid 30s). I have CKD and high blood pressure, also low AMH so it was a miracle. We had already decided before trying that we would not continue if the child was seriously ill. We knew that we would not be mentally, physically or financially capable of raising a child in such a situation or if we had to find ourselves there, it would most probably break us.
At the 12-week scan, the fetus's neck thickness was normal, but other indicators gave a 1:2 probability of Down syndrome. Unfortunately, this suspicion was confirmed in the form of a rare mutation (that a NIPTY test would have given no indicator). It was also suspected that DH had a genetic disorder which meant that all our DC would have DS, fortunately this was not true.
We spent days researching the possible outcomes, which have already been mentioned - the child may have mild mental difficulties, but the condition is more likely to be much worse, and in severe cases, they will spend a lot of time in hospital and/or die young.
So with heavy heart we TFMR at 15 weeks. The autopsy found that the child was relatively healthy physically but it doesn't tell about the mental difficulties.
To our surprise, I was pregnant a few months later. The HCG test results were extremely high and tripled every other day, which is why I was firmly convinced that this child also had DS. However, the 12-week test gave a rather low probability and the amniotic fluid test confirmed that the child was healthy.
DC had to spend a few days in NICU and also in the hospital several times in the first months. Fortunately, these were minor health problems that have now been overcome, and DC is reportedly a healthy one-year-old. This was a very hard time for us as (first time) parents and to be honest the first 9 months in general weren't a plain sailing. We do love our DC to the moon and back but being a parent is hard even if the child is "normal". I can't imagine what life as parents with DS would have been like for us.
I'm glad we were given the opportunity to become parents, but even if it hadn't been that case, I believe we made the right decision for us to TFMR. Only you can decide what is the right decision for you.