I answered that already in this post USB-SSD not waking (spinning) up quick enough, causing read/write-errors
you can make a config backup like in most commercial routers. For installed applications like minidlna the configuration is saved, but the application is not. If you upgrade the firmware you will need to install the application again.
Doing a "system image" like you ask is technically possible https://openwrt.org/docs/guide-user/installation/generic.backup but it's not a standard or roecommended operation so may or may not work correctly on your device as written in that page.