Given a commutative unital ring $R$, we show that the finiteness length of group $G$ is bounded above by Borel subgroup rank one $\mathbf{B}_2^\circ(R)=\left( \begin{smallmatrix} * & \\ 0 \end{smallmatrix} \right)\leq\mathrm{SL}_2(R)$ whenever admits certain $R$-representations with metabelian image. Combined results due to Bestvina--Eskin--Wortman and Gandini, this gives new proof (a generaliz...