Reusing /home is no issue, if you do a little prep work. Best to delete all directories in your /home that contain settings (or better, move those, so you still have them). That includes ~/.cache, ~/.config, ~/.kde, ~/.local and probably you will have a lot of applications specific hidden (a dot in front of them) folders, best to move those too. Easiest to use Live mode for doing this. Then make sure to select the last, manual partitioning option during the install.
No need to set the grub install to boot, you can just use the default MBR option. 90% of the installs os-prober will add Windows automatically to grub. If that did not happen, mount the Windows partition after you boot into your new install (just click on the partition in Dolphin), and run sudo grub-mkconfig -o /boot/grub/grub.cfgthe output will show if Windows is found.
The reason the distro is pacman/makepkg based is exactly stability with about the largest amount of packages available.
See for details. You will have access to close to 50.000 with no loss of any stability since you add by building on your own system.
If you feel though KaOS misses more then a dozen or so of packages you need to have, maintaining more then that might not make sense and you are probably better of with a distro with much larger repos.