Are you planing to build friendlywrt with mainline kernel instead of rockchip bsp? It looks like support for nanopi-r5s board landed in mainline kernel: Nanopi R5S.
FriendlyWrt uses rockchip-kernel, we need to keep up with the updates from Rockchip official. They will periodically release some security patches or fix hardware-related bugs.
Are you planing to build friendlywrt with mainline kernel instead of rockchip bsp? It looks like support for nanopi-r5s board landed in mainline kernel: Nanopi R5S.