Just a quick question. Will you measure with constant current or constant voltage or switch between the two methods depending on range?

As long as you have a suitable resolution on the ADC, then you can keep calibration information in software, and adjust the measured value. But remember that depending on measurement method, you may not get a linear measurement, requiring you to have a mathematical formula or a piecewise linear interpolation when converting from raw measurement to calibrated value.

But it can still be hard to get good readings, because of temperature variations, state of the batteries, moisture, ... so even if you try to calibrate the different ranges, that calibration is only valid around that temperature and with that battery state. Next thing is what to do with ageing. Getting internal resistors of very high stability to automatically compare against?