The present work represents a significant advancement in the design of magnetic nanoparticles for biomedical applications. Herein, an improved chemical approach is presented, involving the thermal decomposition of various Fe-M bimetallic oleates (where M = Mn, Co, and Zn). Through this method a series of nanoparticles (NPs) with moderate doping levels have been successfully synthesized, categorized into monodoped (MFeO) or multidoped (MMFeO). This advanced synthesis technique has yielded six highly monodisperse samples composed of single nanocrystals with an octahedral-like shape and with high saturation magnetization. The uniform composition of the samples has been verified using DC magnetometry, and the dopants' lattice occupation has been analyzed via Fe-Mössbauer spectroscopy. By modeling the AC/DC hysteresis loops, the magnetic anisotropy constants at low and room temperatures have been determined. Furthermore, the biomedical potential of the PEGylated NPs has been investigated by evaluating their magnetothermal performance, magnetic targeting capability, cytotoxicity, and antitumoral therapeutic capacity in a colon cancer-derived cell line. These findings highlight the tunable nature of the synthesized nanoplatforms, enabling precise optimization of their magnetic properties for diverse nanomedicine applications. Notably, Mn-doped nanoparticles have shown efficient heating power at 15 mT, while Mn-Co-doped counterparts have achieved exceptionally high heating at 45 mT. Additionally, the Mn-Zn nanosystem has demonstrated strong potential for both magnetic targeting and magnetic hyperthermia, further underscoring the versatility of these engineered nanomaterials.