PineCube is supported in Buildroot since version 2023.11. See the file board/pine64/pinecube/readme.txt in the Buildroot repository for details how to build it.
The most important thing that this provides is support for the S3’s DDR3 in u-boot. Unfortunately mainline u-boot does not have that yet, but the u-boot patches from Daniel Fullmer’s NixOS repo were easy enough to use on buildroot.
This should get you a functional system that boots to a console on UART0. It’s pretty fast too, getting there in 1.5 seconds from u-boot to login prompt.