Let’s start with the hard truth. The ASUS ZenPad 10 Z300C was not a flagship device. It was powered by an Intel Atom x3-C3230 processor, which was relatively uncommon in the Android world (most devices used Qualcomm or MediaTek). This Intel chipset plays a massive role in the update situation.
This is the most stable custom ROM for the Z300C. It is based on Android Nougat and includes: asus zenpad 10 z300c android update