A 56-year-old male presents with decreased appetite and epigastric pain for 6 months, along with 8 kg weight loss. Hb is 80 g/L, RBC count 3.1×10¹²/L, and reticulocytes 2%. Bone marrow shows hypercellular erythroid lineage with predominance of intermediate and late normoblasts, which are small in size, have scant cytoplasm, and irregular borders. Myeloid and megakaryocytic lineages are normal. The most likely diagnosis is: