I have updated my procedure above with your info. But you now have me curious about making backups.
Thanks!
I just use LuCI to Generate Archive and store the backup file on my desktop pc.
https://openwrt.org/docs/guide-quick-start/sysupgrade.luci
After I've upgraded and reinstalled LuCI and all my packages I upload the backup via LuCI. However, I usually just reconfigure all my settings manually. My setup isn't extremely complex.
Ah... yes of course. (I updated my procedure above to include that advice as well.) I was hopeful that you had some other trick for saving configs.
A while back, I had done some work on using the uci show command to dump certain configs, with the intent of re-playing them in the command line to re-configure. For example, see my config-cerowrt.sh script that I used to repeatably re-flash a router when the firmware was in heavy development.
uci show is a good way to build that script, but it doesn't make for a "plug and chug" procedure that novices can use. Thanks.
I recently performed a reset on my Archer c2600 router running 18.06.2 (squashfs image) to its initial state.
Prior to this I decided to use the opkgscript.sh script to save the list of all currently installed packages & then reuse the script to reinstall all the missing packages post reset.
A big thank you @richb-hanover for making me aware of this script, it has worked as intended when used in this particular manner! 
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However, in the intervening two years, the Attended Sysupgrade package now offers an automatic way to build a new image that contains all the user-installed packages with no manual intervention. Check it out.