Electronic – How to design a new board comprised of multiple already designed sub-boards separated by breakable tabs

The datasheet describes device as very well noise immune one with ratiometric (to power supply) output from DC to 400Hz. So to read DC "ratio" well, try use 2+2+2=6 wire "drive" "sense" cable for 2 devices. At DC you are affected by ohmic, galvanic, thermoelectic forces, but not the radio.

-- Drive --->--*-----+----------------------------+
-- Sense ---<--/     |                            |
                     |                            |
-- Sense ---<----Device (of DC half bridge type),Device--->---Sense--
                     |                            |
-- Sense ---<--\     |                            |
-- Ground --<--*-----+----------------------------+

All 3 sense wires should meet ratiometric input of ADC, say top goes to positive reference input of ADC, middle into positive input, bottom sense goes into 2 pins: negative reference (of balanced input) and negative pin of input.

Edit: Other thought. If there is movement on flexible cable, think about "triboelectricity". If cable friction, hit, vibration is an issue, then read specs about correct impedance of load. If this is high impedance, then add differential "line drivers" with current loop output and match a type of ADC to be "current ADC". Or better add small ADCs on board itself.

You should turn these traces into large copper pours.

You need to co-locate the capacitor with the output of the inductor. That will reduce noise in your circuit, since there is a large AC current flowing out of the inductor into the capacitor at all times.

Consider that the AC path into the capacitor needs to make its circuit back to the ground of the IC, and move it around accordingly.

Make your switching node as small as possible to eliminate noise.C3 looks pretty good, but after that it gets pretty tangled.

You need to add vias under the ground pad of the IC for heat conduction into the ground plane.