Some questions about the image builder and enlarge the rootfs size

Hi all,

i'm build some months ago a image on 21.02.0-RC3 or 4 and i now want to build the official 21.02.0 build, but i have some questions about the last build.

I'm aware that the imagebuilder builds fully inside ram, and i want to enlarge the size if rootfs.
Now the problem is that the i don't have enough ram available to give it the fully 28G. And even if i set it to like 1G the img file become's so large that i cant use sysupdate du of the disksize.
so i tought i build the image, then add some disk space afterwards and compress it?
only problem i dont know how or know the right terms to seach for it or if its even possible?

Is there a workaround for this? Or can i flash the lasted release and keep the kernel settings?

Thanks for your time

Kind regards,