Hi all. Maybe I'll tell you the obvious. But I managed to load the image in emmc.
I installed the official freindlywrt image on the board, connected to it via ssh, mounted a flash drive with the rk3568-nanopi-r5s-ubuntu-jammy-5.19.11-arm64-2022-09-27.img.xz file unzipped in advance and ran the following command
dd if=rk3568-nanopi-r5s-ubuntu-jammy-5.19.11-arm64-2022-09-27.img of=/dev/mmcblk2 status=progress
After the reboot, my system booted up, everything seems to be working.