For what it’s worth, a large number of the things you listed are actually portable into Sway, i3wm, and a lot of other tiling wms just by way of running the KDE settings daemons - I do the same kinds of things (network printer, theming, auto-mount, auto-start, XDG config, firewall, vpn, network settings, monitors, keyboard layout) just by having i3wm start up xfce-settings-daemon.
I’m not familiar enough with KDE to make promises about grub and splash, but I would imagine those would also work exactly the same as well. In fact, a little bit of searching and it looks like if you’re on Wayland you could even just replace KWin (the KDE window manager) with Sway in the startup files and be 95% of the way there. Might just need to configure a system bar or something to that effect.
