A couple suggestions then:

• I have a LabJack that I use for things like this. I’d put a sense resistor on the high side, tie that into one of the differential inputs, set up the (admittedly awful) software, and boom I’d have battery voltage and current over time. But that’s several hundred dollars.
• Instead of that, you can homebrew a pretty similar thing.
  • Either use your Pi or a separate Arduino to read voltages. Store it in a file or send it to a host PC over serial every [1-3600] seconds
  • You could get a module like this one to capture everything digitally
  • Or you could do it the analog way
    • Probe battery voltage with an appropriate resistor divider
    • Probe current with an INA169 or similar
• If you are using a BMS, that could give you net power flows in/out of the battery. But you won’t know from that alone whether your solar setup is functioning well or your power draw is low (for example)
• For battery state of charge, you could worry about coulomb counting and fancy algorithms. I’d just use voltage. But know battery capacity is not linear with voltage (refer to charge/discharge curves).