Tunability of magnetic properties in the Ni-intercalated transition metal dichalcogenide <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>NbSe</mml:mi> <mml:mn>2</mml:mn> </mml:msub> </mml:math>

We study the magnetic and electronic properties of Ni-intercalated ${\mathrm{NbSe}}_{2}$. We calculate the magnetic exchanges of ${\mathrm{Ni}}_{x}{\mathrm{NbSe}}_{2}$ ($x=1/3,\phantom{\rule{4pt}{0ex}}1/4,$ and 1) and find that the out-of-plane magnetic coupling depends on the Ni connectivity: it is ferromagnetic when Ni atoms stack on top of each other and antiferromagnetic otherwise. Focusing on ${\mathrm{Ni}}_{0.25}{\mathrm{NbSe}}_{2}$, we identify a ground-state transition from a stripe antiferromagnetic phase with Kramers degeneracy to a ferromagnetic phase above a critical Coulomb interaction ${\mathrm{U}}_{C}$. Spin-orbit coupling lowers ${\mathrm{U}}_{C}$, aligns the easy axis along $z$, and stabilizes collinear antiferromagnetic and ferromagnetic states over the competing ${120}^{\ensuremath{\circ}}$ phase. Ni intercalation also strongly modifies the electronic structure, replacing the $\mathrm{\ensuremath{\Gamma}}$-point hole pocket of pristine ${\mathrm{NbSe}}_{2}$ with an electron pocket and shifting the Van Hove singularity away from the Fermi level, thereby suppressing potential instabilities. Finally, we investigate the altermagnetic phase in the broader class ${\mathrm{T}}_{0.25}{\mathrm{MX}}_{2}$, finding that spin-orbit effects induce orbital antiferromagnetism with weak ferromagnetism or ferrimagnetism depending on the N\'eel vector orientation. Our results demonstrate that Ni-intercalated ${\mathrm{NbSe}}_{2}$ provides a versatile platform to explore and tune multiple competing magnetic phases that lie close in energy.