Now the main question is,
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?
Because I wanted to run Bitcoin LND node after a few days.
Backup Bitcoin file to another drive. When you are done with new installation, copy the file and put in the right directory and your Bitcoin core will run without doing another initial block download but you may have to sync with to the current block depending on how long you went offline during your new window installation.