Another copy. Would have been crazy if it was the exact copy I had.

I was at a used bookshop the other day and found the same Caldera Open Linux 2.2 book and cd that I used to install my first linux distro on a pc. Man that was exciting!

I’m no expert so take this with an appropriately sized grain of salt.

You should be able to install KDE on whatever distribution you decide. If you want KDE 6, you may have to add a repo, but it should be as simple as sudo <package manager install incantation> whatever-KDE-is-named-in-the-repo

If you want stability, Debian is the go to, but the tradeoff there is older packages. However if manjaro is working for you, don’t fix what isn’t broken. I don’t know how good Debian is for gaming, but honestly any distribution should be just fine for dev. Considering what steam has done with Arch as the base, it may be worth considering Arch as an option.

To the partitions, I’m not knowledgeable enough to make recommendations as to what you should or shouldn’t touch. My instinct is to not touch /boot/efi

Something can definitely go wrong when playing with partitions, so make that backup of everything as planned and test it before you make any changes to the system.