I did some preliminary testing to find out if I can replace the Arduino UNO with a less power-hungry Arduino Pro Mini.

PWM signal generation on pin D3 - works

Reading power values from INA3221 through I2C - works

The Pro Mini doesn't really have to power an LCD in the long run. If it can do that, great; If not, no problem. I don't really want to waste power on an LCD for a solar charge controller anyways. So far, this is looking pretty good.