For most, the best "custom" experience is simply optimizing the stock software. You can enable to tweak animations and networking settings, or use the My Huawei app to ensure you're on the latest security patch.
Use fastboot to flash a recovery image like TWRP specifically built for your model: fastboot flash recovery twrp.img .
Huawei MatePad 10.4 Custom ROM Guide: Current State and Alternatives As of April 2026, the custom ROM scene for the Huawei MatePad 10.4
To change the look and feel of the device (often the main goal of a custom ROM): Third-Party Launchers Nova Launcher Niagara Launcher to replace the default Huawei home screen. Revamped Multitasking