Right, now get a borderline computer-illiterate person to connect to your network, ensure their firewall isn't misconfigured to block all incoming traffic (with TeamViewer, this configuration would still work because the device just connects to the TV server) and open and set up a completely separate screen sharing program.
I know none of these steps are difficult if you have any idea what you're doing, but I've met plenty of people who would most likely need assistance going through the motions. Funnily enough, the best way to do it remotely would probably be to get them to install TeamViewer to then set this up for them remotely.
By the way, as far as networking goes, Tailscale does the same thing TeamViewer does, just for a VPN instead of a screen sharing application - it will try to do all the NAT punchthrough techniques and IPv6 connection and fall back on tunneling through relay servers if all else fails. It's not any more of a direct connection than TV.
xrandr is Xorg only, it doesn't work with Wayland. You should be able to make SDDM use your Plasma display configuration - https://wiki.archlinux.org/title/SDDM#Match_Plasma_display_configuration
No clue if that's going to fix your issues, but at least it's supposed to work with Wayland.