So searching for devices with Vn <= 4nV/rt(Hz), and In <= ~100fA/rt(Hz), I find a couple of options:
ADA4625, previously mentioned
3.3nV/rt(Hz)
4.5fA/rt(Hz)
max Av=90V/V for BW=200kHz
Iq=5mA
AD8655 2.7nV/rt(Hz)
ADI parametric search shows 1fA/rt(Hz), but then that spec is nowhere in the datasheet. Their forums show other people asking, but no official answer besides "10s of fA/rt(Hz) around 10kHz, up to low hundreds of fA/rt(Hz) at lower frequencies" (
https://ez.analog.com/amplifiers/w/documents/1686/ad8655-current-noise-density)
max Av=140V/V
Iq=5.3mA
OPA607 (pre-release part from TI)
3.8nV/rt(Hz)
47 fA/rt(Hz)
max Av=250V/V
Iq=1mA
OPA828 4nV/rt(Hz)
1.2 fA/rt(Hz)
max Av=225V/V
Iq=7.9mA
OPA827 3.8nV/rt(Hz)
2.2 fA/rt(Hz)
max Av=110V/V
Iq=6mA
It seems like AD8655 is the best of the bunch in terms of voltage noise, but without any sort of characterized spec on current noise density, I'd be hesitant to use it. So then, ADA4625 looks like the best realistic bet. All the other op-amps/instrumentation amps with lower voltage noise seem to all have much higher current noise densities that wipe out the gains, based on the impedance of my sensor.
If I were to use ADA4625-2 (dual version), use the first amp to get all the gain, and then second amp to invert that amplified output, so signal chain is fully differential after that point, the benefit would be in rejecting any common mode noise coupling into signal chain. Is there a realistic benefit to that approach, or is maintaining single-ended until I get to a fully-diff-amp to drive ADC inputs a more realistic topology?