Nice troubleshooting, well done! In your post, you should have a button to mark it as the solution to this thread. That will help others to find it quicker and for reporting purposes.
The topic of making backups is a big one in and of itself.
For general backups you can use Rescuezilla which will create bit-for-bit copies of your entire drive, but this is something that should only be used for things like catastrophic failures given they take a long time to make and restore from.
Then there are tools like Timeshift, which you can use to make snapshots of your system. A snapshot is not a backup per se, rather they are like checkpoints in a video game that allows you to restore your system as it was at a given point in time. Snapshots are great for testing things like installing new software that may cause issues, or even system upgrades, but they shouldn't be used for personal files.
As for personal files, a backup in this sense basically means creating copies of your data on a separate location. A common backup strategy is the 3-2-1 rule. There are tools that you can use to make this easier like Deja Dup, already installed in Zorin OS.
To uninstall Windows completely, the best way is to install Zorin OS from scratch and instruct it to use the entire drive. This would override the bootloader entry for Windows and reclaim the entire disk space. In fact, depending on your drive's partition layout, this may be the only way to do that. You can see your current partitions using Disks to take a look at it.
But it would be best to open a new thread if you have any questions about any of these topics. Or even if you want to start a discussion about browsers, backup strategies and whatnot. Use the appropriate categories for that: General Help for assistance, Chat about Zorin for discussions, Feedback if there's something you'd like to see improved or share your experience, etc.