I suspect that the existing filesystem is not fixable at this point. You should be able to get a larger filesystem after re-installing an image to your sd-card and first boot.
Since you are using image-builder, you probably can build the image with the larger filesystem. See this thread specific to the R2S for info: ImageBuilder - How to create an image with a custom filesystem
It looks like some people have had success with expanding the second partition before first boot and the writable overlay filesystem gets created larger at first boot.
If you want to continue with the expanding method, I have a few thoughts to share.
I suspect the errors you ran into may have been caused by using the wrong tools for the filesystem you have, you had the wrong offset for the loop device you created or you pasted all the commands together rather than one at a time and waiting for the prompt (or error message) between each command. In the set of commands in the method I shared and you linked, you need to type them as-is and don't try to substitute values for variables yourself.
I have an R4S and not and R2S but I think others have said they use the same basic storage layout.
If you want some specific help, please reply with some info from the R2S after re-installing your image and booting it:
Output of df -hT
Output of losetup
Good luck!