Hi Solar, putting current shunts before the regulators would show how much current each string is putting out. but would not show how much each string is feeding into the batteries. As the MPPT controllers convert high voltage low current into lower voltage higher current.
It would be best to put the shunt in the negative line after the regulators that would tell how much current each regulator were feeding into the batteries.
I have three MPPT controllers on my system, I monitor the total current on the outputs but also have a fairly cheap Clamp Meter that I can check to see what each regulator is doing.
My system is pretty simple, works great, even on cloudy days ( which we have plenty of) we still get enough power.
We don't have big power usage though. No air conditioning, wood heating, 12 volt lights, 12 volt fridge, and power everything we can without the inverter. We only turn the inverter on when we need it. So our 2kw system is more than adequate for us.
Have fun with being retired. when you get there, it is a great time of life. I really can't figure out how I had the time to go to work when I did.
Pete