Is there any (free) way to use a (Samsung Galaxy Tab S5e) tablet as a second monitor (with Windows) without an app? But if that's the only way, then that'll do.
Yes, I am using spacedesk, but it isn't super smooth enough for watching movies, enough for working and coding. It is available for Android and Apple devices.
You will need to install the same software on Windows. Windows acts as a server. Open the app on your Android, and connect to the same WiFi (it says even wire can be connected).
Your laptop's software has to be turned on to make it work, but it works well
If you have an Android phone, you can connect that as well. Once everything is set up, don't forget to change the display option in Windows settings.
Hope this serves your purpose.