For the RK3229, "Custom ROM" usually refers to optimized stock firmware or community-modded Android builds (mostly based on Android 7.1 Nougat or rarely Android 8.1 Oreo).
Flashing custom firmware on RK3229 devices requires specific software and hardware setups: Generic RK322X (rockchip-rk322x) - postmarketOS Wiki
The Rockchip RK3229 is a quad-core processor based on the ARM Cortex-A7 architecture, typically coupled with a Mali-450 MP2 GPU. While it was a capable chip for 1080p video playback in its prime, it has significant limitations by modern standards: