What resistor should I put in series with an LDR for minimum current draw when using an Arduino Nano?
LDR’s are very sensitive but also very inaccurate for tolerances.  Otherwise PD’s are less sensitive but very accurate for tolerances like 0.5mA/mW.
The optical paths used to detect solarity angle error are just as important to block diffused cloud signals. You may want to consider wide and narrow beam width detectors for wide diurnal corrections or time of day predictive corrections.  A narrow beam width can be created from a wide angle PD by using black heat shrink over a 5mm PD from Sharp/Vishay or Infineon PD from Mouser. Geometry allows you to define your angle from this goal.
The impedance you choose for sensing current on a reverse biased PD depends on the EMI induced currents in the area of the wire loop to the sensors. Thus STP cable is preferred for shielded twisted pairs grounded at one end only to avoid sheath current coupling to the twisted pairs. 10k is reasonable but 100k is possible to attenuate motor noise with a shunt cap to choose the delay time and attenuation of noise.
Orientation of cables at right angles (orthogonal also helps reduce coupling from stepper or servo gear motors used for tracking. Since solar tracking does not demand much BW so RC filtering is possible to increase the SNR and reliability of a differential solar angle detector with careful selection to avoid latency and overshoot in the control.
It is also useful to consider using the PD output to correlate with the Voc of the PV array. from this it is possible to regulate charger in a predictive tracking way so that the converter impedance results in a PV voltage from 83% to 72% of the Voc without having to disable the PV to measure it or use a MPPT hunting algorithm or IC to obtain maximum power transfer PV voltage vs solarity. it can also be used to detect if your PV’s are dirty if they do not correlate with your sensors.  Matching the PV impedance is what really defines the MPT operating point. For any given PV acting as a current source, the impedance of Voc/Isc is equate to the Vmpt/Impt for any variations due to dirt , solarity and size.  This can be correlated with your PD design by calibration for the use of tracking and MPT predictive power conversion,as a good approximation in a rather simple solution.  SMPS input impedances are complex and time-variant but with careful design you can optimize power conversion for charge control, so that you regulate the load impedance and the CV/CC/cutoff to your storage batteries and optimize energy capture.
Always start by thinking how to verify your assumptions with a list of specs , toler@CES and variables, then plan on verifying each assumption during the design phase. we call this DVT.