RK3588 is also in the pipeline for MNT Reform. Originally I wanted to go with Radxa CM5 but I haven't heard from them again since some initial emails. I have a similar SOM from T-Firefly but it will need a custom adapter, which I have to start designing soon.
One caveat: Valhall GPUs such as the one in RK3588 need binary firmware for the embedded ARM in the GPU.