Quantum magnetometers based on spin transitions of solid-state point defects can achieve excellent sensitivity and spatial resolution at ambient conditions, in a convenient, stable material platform. The diamond nitrogen vacancy (NV) center has dominated this field, but several different atomic defects in silicon carbide (SiC) provide similar advantages in a host material far more amenable to large-scale growth, fabrication, and integration with existing semiconductor technologies. We explore the negatively-charged, h-site silicon vacancy (V2) in 4H-SiC, which, like the diamond NV center, is an intrinsic defect, with an intersystem crossing allowing for optical initialization and readout of a spin-3/2 ground state at room temperature [1-2]. However, lower optical contrast and shorter coherence times prevent it from equaling diamond-NV magnetometers. To fully employ this defect it is necessary to understand the limiting dephasing sources and extend its coherence times. Here, we compare V2 ensembles created by electron irradiation in natual-abundance (29Si = 4.7%, 13C = 1.1%) and isotopically-purified (29Si = 0.01%, 13C = 0.15%) SiC epilayers. By reducing spin-spin interactions with the nuclear-spin-carrying isotopes 29Si and 13C, both inhomogeneous (T2*) and homogeneous (T2) dephasing times are improved by more than a factor of 10 [3]. With this dephasing source suppressed, T2* and T2 decrease with increasing irradiation dose, showing that irradiation-produced effects are now a limiting factor. At the lowest doses, T2 approaches its spin-relaxion (T1) limited value. We also find that T2* and T2 vary with magnetic field strength (Figure 1a-b), the former due to state mixing at the spin level anticrossings, the latter due to electron spin echo envelope modulations (ESEEM). ESEEM arise due to interaction with spin-carrying nuclear isotopes [2], and we explore how their behavior changes in natural-abundance vs isopure SiC. Finally, we continue to explore ways to make full use of the different Δms=1 transitions available in this spin-3/2 system. Simultaneously monitoring both {1/2, 3/2} transitions can effectively double our magnetic-field sensitivity. Meanwhile, the {-1/2, +1/2} basis is insensitive to shifts in the zero-field splitting (ZFS), such as those due to static strain or electric fields, resulting in a longer T2* but no change to T2 (Figure 1c-d). Benefitting from both isotopic purification and careful implementation of this choice of bases, we demonstrate DC magnetic field sensitivites as low as 4 nT/√Hz [4] and expect to achieve few-hundred pT/√Hz. This work was supported in part by the U.S. Office of Naval Research, the Defense Threat Reduction Agency, and an appointment to the NRC Research Associateship Program at the U.S. Naval Research Laboratory administered by the Fellowships Office of the National Academies of Sciences, Engineering, and Medicine. [1] P. G. Baranov, et al., Phys. Rev. B 83, 125203 (2011). [2] S. G. Carter, et al., Phys. Rev. B 92, 161202 (2015). [3] I. Lekavicius, et al., PRX Quantum 3, 010343 (2022). [4] I. Lekavicius, et al., Phys. Rev. Applied 19, 044086 (2023).