I'd do a test and see if the mains (normal-mode) hum rejection is actually working. Should be a zero-cross detector on the board so the DMM can figure out line freq. as its predecessors did.
The DMM seems to spaz out way too much? I also don't find these Keysight bench multimeters (34461a, 65a) a good drive. The readings are all over the place, so much flicker it's like a slot machine in a casino to read them even with a dead quiet source like a battery. It's like it's picking up tons of strange noise that actually isn't external to the DMM.
Another test I would do is look at the common-mode noise the DMM has at the (-) input, with a scope or spectrum analyzer. You can a put a 10nF from (-) input to earth-ground to stop HF CM noise, assuming it doesn't already have that as a Y-cap from the floating A/D to PE.
I was disappointed to see Keysight copied ANENG by using Hongfa relays
and a 10k resistor across the coils seems weird.
A problem is the DMM looks expensive to manufacture - PCB with SMT on both sides, through-hole and flying leads, tons of manual assembly with the AC mains wiring, fuse etc. and a kilogram of shields over shields.
Instead of lowering cost by using cheaper components, a cost improvement project would first lessen manufacturing labour costs instead of relying on china for assembly.
Recycling the old enclosure and sheet metal saves development costs (NRE) but in the end it might add too much fat because it's not ideal - the power transformer +wiring are sources of hum but poorly shielded from everything and not away from the analogue board.