Do not waste time searching for custom ROMs. Use the tablet as a basic video player, e-reader, or smart display. If you need a modern Android version, recycle the Z300C and buy a used Samsung Tab A (2019 or newer) which has official LineageOS support.
This is an interesting topic because the — codenamed Frost — uses an Intel Atom x3-C3200 (SoFIA) processor, which makes custom ROMs extremely rare and difficult. Asus Zenpad 10 Z300c Custom Rom