Customising an Asus tablet requires specific prerequisites to avoid "boot looping" your device:

: Unlocking is the first required step, but Asus has historically discontinued support for its official unlock tools for older Zen devices Intel Atom Architecture

No major ROM sites (XDA, LineageOS wiki) officially support this tablet. However, you can find community projects here:

Flashing a custom ROM on a standard Snapdragon device is often a weekend project. Doing so on a Zenpad Z300C is a rite of passage. It involves unlocking the booloader, navigating the terrifying black screens of an Intel-based fastboot, and praying you don't brick the partition table.

: Modding your device carries the risk of "bricking" it and will void your Asus Warranty . Proceed only if you are comfortable with technical troubleshooting. Builds for Lineage Supported Tablets - UnOfficial Builds