I think VNC protocol over your tailscale tailnet would work well. It shares the screen without locking out the in-person user.

You could also try Steam remote play, it shares the desktop like what you’re asking, but would require a bit if setup (custom “game” added which would expose the entire desktop for e.g.), and wouldn’t be useful without someone relaunching it after a reboot, so wouldn’t be very useful for remote management without grandma available at the machine.

I would stay away from X11 over SSH because X11is deprecated in favor of Wayland and will stop working as you upgrade the OS (if it even works now).

RustDesk looks promising but it sounds like it may lock the screen for the in-person user, but I’ve never used it so can’t say for sure.