Here's a study, it depletes iron levels and can take a long time to recover, maybe more so in our age group.
"Adverse events and retention of donors of double red cell units by apheresis"
Adverse events and retention of donors of double red cell units by apheresis - PMC
Discussion
2RBC donation is overall safe. Donor retention was low over a period of 11 years. An important reason for abandoning 2RBC was the detection of low SF (iron) levels. The impact of fatigue on donor retention and the course of iron stores after repeated 6-monthly 2RBC apheresis require further investigation