At least for status monitoring (low bandwidth keep-alive packets), there are other options available. LORA (and/or LoraWAN) can bridge quite big distances line-of-sight. And there is the possibility of using something like an Arduino with a RockBLOCK Iridium sat module to send a daily "i'm still here" message that includes the battery charge status, if you can read that out from the solar charge controller.

The Arduino also could monitor the logger itself and basically press the reset button if the logger has crashed.

As for the power situation itself: The 60m cable is a big resistor and a decent amount of inductance as well, i suspect. If the logger pulls more power momentarily, the voltage will drop very sharply for a brief moment, then possibly rise above safe levels when power usage drops due to the inductance.

The voltage instability problem will be possibly be worse if the power cable is buried in the soil, especially if it's wet, because on every change of power usage, this will change the voltage in the cable and the soil would act as the other end of a capacitor.

Therefore, the voltage booster needs to go on the end of the cable where the logger is, with extra input capacitors. Making sure it can momentarily handle ovrevoltage as well and regulate that down. It should be a combined buck/boot converter.