For starters, I got as far as successfully reading low-ohms accurately. But once I get into 10K or 100K ohm range, accuracy starts to suffer, and it's utterly useless for mega-ohm ranges. Which is what I need to measure.
I'm currently using the method of two resistors, one known and one unknown, aka a "voltage divider", measuring the voltage drop via one of the analog pins and doing the math. So I have the fundamentals working fine. But apparently there are esoteric things going on when you're trying to read mega-ohm range values which require more stuff to be added that I'm afraid I don't really understand yet.
A couple posts mentioned an op-amp "voltage follower" but without elaboration or demonstrating how to incorporate it. Basically I'd like to take this thread: https://forum.arduino.cc/index.php?topic=635576.0
...and pick up from post 5 (OP went AWOL).
One question I know you're going to ask me is what the range is I need to measure. And I'm afraid I don't really know yet. I'm reading resistance off a pressure sensor, and the resistance goes down as more pressure is applied. I don't yet know the upper limit of what the pressure is I'm going to be faced with. So far my calibration tests have produced resistance as low as about 1 mega-ohm with 26Kg of pressure. 4Kg resulted in 8.5 mega-ohms. I have a power curve formula already worked out which approximates Kg based on ohms (fitting the function to an X/Y scatter graph of my assorted calibration readings) sufficiently accurate for my needs, so I just need to read the ohms relatively accurately.
So I feel I've worked out 90% of the hard stuff, if I can just have some guidance on the last 10% to bring the two ends of this together.