If I install Windows 10 on my PC, do I have to download the entire Bitcoin Core again?
Or if I want, can I backup my Bitcoin Core files to another drive and restore them later?
You can backup your datadir to an external drive so that when you reinstall Windows 10, you can simply reinstall the Bitcoin Core program and copy the datadir back to your internal drive.
Since your node is pruned, it should even fit inside a USB stick, so I don't think you will have to go out and buy any additional hardware.
How did Windows 11 decrease the lifespan of your SSD though? I am confused about that.