  We propose an implementation of a singlet-only spin qubit in a GaAs-based
triple quantum dot with a (1,4,1) charge occupation. In the central
multi-electron dot, the interplay between Coulomb interaction and an
out-of-plane magnetic field creates an energy spectrum with a tunable
singlet-triplet splitting, which can be exploited to create a six-particle
singlet-only qubit with a qubit splitting that can straightforwardly be tuned
over tens of $\mu$eV by adjusting the external magnetic field. We confirm the
full exchange-based electric control of the qubit and demonstrate its superior
coherence properties due to its singlet-only nature.
