Max theoretical esp32 chip UART speed is 5Mbps, but nobody can reach this speed - max "config speed" is 921600 and even on 460800 you can get garbage and data loss 😞
For RockPi you need 1.5Mbps. I don't know why Radxa need such speed, may be they think "fast is better"
I use BeagleBone single board computer for connecting to my RockPi3A (because HDMI not working in modern kernels), and it is working perfectly. BeagleBone has fastest UARTs avaliable in pocket size