Since we are talking about our NanoPi R5S/R6S images, replacing the bootloader is mandatory to make it use any kernel from the rootfs. When this is done, you can also use e.g. the kernel builds from Armbian (and their U-Boot builds as well), who also offer some based in mainline Linux, running very well at least on R5S/RK356x.
I want to test removing the first 7 partitions from our images, so that the rootfs is the only left one, and then flashing the Armbian U-Boot. If this works, then probably the rootfs/partition can additionally be expanded to use the leading 140 MiB which the 7 bootloader/kernel/config partitions did previously use.
I’ll start working on this once I’m done with some RPi 5 image generation and testing.