Swapfile for Kobo:
1. make a blank file, your choice in size.
dd if=/dev/zero of=path/to/your/swapfile
2. mkswap path/to/swapfile
3. place on device
4. using some sort of terminal,
i recommend storing this in your book partition (user storage) for easy backup, swapfiles can work from anywhere pretty much. I keep mine in my book partition because its too big for the rootfs lmao
note that I didn't tell you how to make it auto mount, which will make it stay between reboots. I don't know how to make it automount and honestly it doesn't really need to for most things. I just use NickelMenu shortcuts to enable and disable it
Mount your Kobo on your computer, go to the .kobo folder (make sure the ability to see hidden folders is active), make a folder called "screensaver". Put anything jpg that you want in there, doesn't need to be greyscaled manually unless it royally fucks up with a color version. On the Kobo side of things, activate showing current read and showing book cover on the sleep screen.