Is it possible to do a full backup of my installed openWRT, that I can use with sysupgrade, to restore all of my packages and configs?
Short answer, no.
Any attempt would require you to do the development and is quite risky of bricking the device, as there are quite a few things to consider.
Take a look at the Attended Sysupgrade process... it's generally what you want, and is a much more practical approach as compared to the idea of a 'full backup/restore'.