We evaluated various heat treatments (HT) for maximizing the Nb ₃ Sn layer thickness while retaining a refined grain microstructure in low filament count internal-Sn Nb ₃ Sn Rod-In-Tube wires with internally oxidized nanoparticles. These wires were manufactured in our laboratory using SnO ₂ as oxygen source and Nb alloys containing Ta and Zr or Hf. By reacting the wires at 650 °C for 200 hours we obtained relatively thin reaction layers but high layer critical current densities (layer J _C ) of ∼3000 A/mm ² for Hf-containing wires and ∼2700 A/mm ² for Zr-containing wires, both at 4.2 K and 16 T. Notably, both of these values are over the layer J _C threshold of 2500 A/mm ² , which is estimated to be necessary for attaining the corresponding Future Circular Collider (FCC) target non-Cu J _C of 1500 A/mm ² . Following this heat treatment, the fine-grained Nb ₃ Sn area occupies only ∼35% of the filament area for Hf-containing wires and ∼20% for Zr-containing wires. After heat treatments with a reaction step at 700 °C these values increase to 70–80% and ∼60%, respectively, with only a minor increase of the grain size. However, we observed a noticeable decrease in the layer J _C for these HT. Magnetic measurements show that the high J _C wires exhibit a point defect contribution from precipitates to the pinning force, which is missing in wires with depressed J _C values. The higher heat treatment temperatures may have caused excessive coarsening of the oxide precipitates, to sizes unsuitable for flux pinning. Reaction heat treatment temperatures in the range of 650 °C to 700 °C and durations between 50 and 200 hours may provide a better compromise between the Nb ₃ Sn layer thickness, its grain size and nanoparticle size.