I found a few items of interest on Google searching string “ltspice solar panel model”. From what I saw, they commonly use some combination of a current source and parallel diodes (and possibly a BJT controlled by the current through the diodes to mimic a solar panel. Then, by creating a time-variant current source, you can simulate irradiation conditions.
Regarding the LT8390, it cannot implement MPPT without external control. MPPT can be implemented in a number of ways, using a programmable device like a microcontroller or via various analog topologies. One would use the ISMON signal and either the direct voltage of the solar panel or, more likely, a divided down version of it as your variables to solve for power, and you would drive the CTRL pin appropriately to control the load current of the LT8390. One would then have to use an algorithm to vary the load and compare power with the previous results. However, I am not sure how one would characterize these within LTSpice.