Isolated BNC connector in metal chassis, EMC considerations
Hard ground is preferred for coax because the impedance are low (50,75) and good coax is a must where semi-ridged  coax is preferred for critical circuits in the GHz range. ( used by all and HP spectrum Analyzers and sub-ns  scopes).
But you implied avoidance of Tri-ax connectors implies high impedance audio sources.  Therefore balanced source impedances are best to reject common mode noise.   Depending on the spectrum of interference also demands a range of solutions.  High L CM chokes at source for line noise and low L CM chokes for RF with an RF shunt cap to make a Pi filter.
In the old days, signals often went thru capacitive shunting feedthru’s for cable signals into a box, and RF AM can get demodulated if sufficiently coupled into high impedance preamps from long cables which need to be shunted by an RF cap near entry.
Details require the source, cable and load impedance over the entire spectrum of signal and noise.
If all the signals are very long wavelength compared to cable length then cable impedance and reflections won’t occur on the signal but maybe on RF noise.