Yes, I would skip that for now. It's tedious so... try the easier, first.
Also, in terminal, you can run
apt list --upgradable
To see if any packages can be update.
You can run
sudo apt dist-upgrade
To upgrade (update) any packages that do not require deletion of files to be upgraded.
Or if you are agreeable to packages being upgraded even if deletion of old files is necessary (99% of the time this is safe)
sudo apt full-upgrade
Honestly, I always check for updates and do upgrading through the Reliable Terminal.
You might test in terminal if upgrades are available and run
sudo apt dist-upgrade if there are any packages listed as upgradable. If it works in terminal, then that should narrow the problem down to just Gnome-Software.
If you are unable to do so in terminal with APT, then we need to troubleshoot other things. So far, your sources.list looked fine.