A little late and I can't remember the full details, but I did go through kindlemodding dot org. I suspect initially I couldn't get WinterBreak2 working on an unregistered kindle. IIRC you have to reboot after setting up wifi to skip the registration and get to the browser, but I didn't realize that at first.

I ended up installing LanguageBreak but it took many attempts and many many reboots of the kindle into demo mode which takes forever. Even now searching how to jailbreak an unregistered kindle Paperwhite of my generation (can't remember off the top of my head) top results point to using LanguageBreak.

Once I got WinterBreak2 installed it was pretty easy. The site mentions a lot of stuff about package managers that seem to not exist at all which was confusing.

Now I'm happily booting straight into KOReader using an init script that I wrote that is gated on the presence of a file in the USB storage area.