  We present a pair of seismometers capable of measurement in all six axes of
rigid motion. The vacuum-compatible devices implement compact interferometric
displacement sensors to surpass the sensitivity of typical electrical readout
schemes. Together with the capability to subtract the sensitivity-limiting
coupling of ground tilt into horizontal motion, our seismometers can widen the
sensing band towards mHz frequencies. This has notable applications across a
range of fields requiring access to low-frequency signals, such as seismology
and climate research. We particularly highlight their potential application in
gravitational-wave observatories (LIGO) for observation of intermediate-mass
black holes ($\sim 1000\,M_\odot$). The sensors are based on a near-monolithic
fused-silica design consisting of a fused-silica mass and fibre, showing
improved stability and robustness to tilt drifts, alignment, and control
compared to all-metal or mixed metal-silica designs. We demonstrate tilt
sensitivity that surpasses the best commercial alternatives in a significantly
reduced footprint compared to our previous iterations of these sensors.
