When you connect to a Wifi, network manager stores all connection details inside /rw/config/NM-system-connections/[wifi].nmconnection.
Just copy that file to the hopefully dedicated template of your disposable sys-net and your disposable sys-net will work with it on next boot.
The same applies for all the other modifications that need to remain persistent. Do them inside the template. You can see from mount what is persistent in your template (everything on /dev/xvdb).