Sorry for you loss. From what I read during my MMC in the summer there isn't any evidence that supports breastfeeding leading to an increased chance of miscarriage. For this reason I would not be put off TTC whilst breastfeeding.
All my pregnancies excluding the first were conceived while breastfeeding. Of these I have had 1 singleton live birth, 1 MMC of a embryo measuring 6+3 and a twin pregnancy consisting of a viable pregnancy and blighted ovum.
I did find one area where I think more research would be useful... I don't have the links to the research support this handy any more and its a bit in depth...... You know how older women have an increased chance of miscarriage, as well as an increased risk of trisomy 21? And that the majority of early miscarriage's are caused by genetic abnormalities, the most common of these being a trisomy?..... They did some research on women of all ages with just one ovary and found these women where also at an increased risk of babies with trisomy 21. They concluded this was due to higher levels of FSH. Higher levels of FSH means the body is working harder to ovulate and may end up picking a less than perfect egg to release, which may have another trisomy incompatible with life leading to miscarriage.
With me so far? So with breastfeeding you have an increased chance of fraternal twins. This is because your FSH levels are higher. So if higher levels of FSH can cause an increase in trisomy 21, which is similar to trisomy's that can cause miscarriage..... and breastfeeding means that you have higher levels of FSH.... then it stands to reason that breastfeeding would mean a higher chance of miscarriage due to fetal abnormality.
However there is no research to support my hypothesis. Perhaps because there isn't a connection, perhaps because no one has thought to do the research, perhaps because the numbers of women conceiving whilst breastfeeding is too small to make it worthwhile research.