how to cross compile 4.19 BSP?

[Philip C]

I am sorry if this list is not exactly the right place for this 
question, but I don't know where to ask.

Until mainline supports the DSI/LCD output I need something that I can 
use for my project based on an RK3566 with an LCD screen.

So I have been trying (and failing) to compile the 4.19-develop branch 
of https://github.com/rockchip-linux/kernel

I have been using this cross compiler:-
gcc-arm-10.3-2021.07-x86_64-aarch64-none-linux-gnu

using these commands:-
export ARCH=arm64
export 
CROSS_COMPILE=/home/philip/gcc-arm-10.3-2021.07-x86_64-aarch64-none-linux-gnu/bin/aarch64-none-linux-gnu-
make Image modules
make dtbs

This works on other kernels, but with the official kernel source I get 
many, many errors which are probably beyond my abilities to fix.  For 
example I can compile Radxa Team 4.19 kernels, but my project isn't a 
Radxa product.

Three questions:-

1. Should I be using branch 4.19-develop? or 4.19-develop-rt87?
2. Is there a particular OS/version and cross compiler that I should be 
using?  Currently I am on ubuntu hirsute but I have no problem to change 
it to something else.
3. If this isn't the right place to ask then please where is?

thanks, Philip