Our absolute favorite iron supplement is Ferrasorb by Thorne. It actually gets absorbed and has a few other ingredients to help build bone marrow and blood. No GI upset.
My dh has chronic anemia and we've tried them all.
Thorne Research - Ferrasorb - Complete Blood Building Formula with Iron, Folate, and B vitamins - 60 Capsules
https://www.amazon.com/dp/B01IE1SG8E..._Mr8dzbPGXJC8G