Try this:
1)use opamp opa335 or LMP2011 or dual versions of these(1..2USD Digikey) .
These are "autozeroing".
2)There are differences in circuit implementation, regarding
what Your ADC can handle.
If You need differential pair to ADC differential input, then
use opamp's_output and one of sensor output as pair(try which sensor output). Typically hall effect are high impedance, so here maybe is naccassary to buffer this sensor output with additional opamp. On primary opamp You can adjust offset with
potentiometer (1-vcc_R,3-gnd_R,2 - trough resistor to non-inverting input of opamp). Connect pin1 of potentiometer to VCC
trough resistor (example-10k), pin 3 - to gnd trough 10k,
to opamp - 4..10k, and use 1k potentiometer.
This works for me in case of load cell (350 ohm).

Maybe 2opamp cross connected as inamp can be "offsetted" too
but i was not trying.