The Ubuntu developer who looks after the ubuntu nvidia-prime package has updated his work for Ubuntu 18.10 (currently between beta and release).
It works with lightdm and the non-wayland session. Swapping between hybrid and intel-only (either way) works without rebooting, you just need to log out & in again. The nvidia card is turned off with acpi calls, similar to bbswitch although the code is incorporated into a supported ubuntu package now, bbswitch is not a dependency.
Also, mode setting and prime synchronisation is active by default; I think this is the first time a distribution has done this.
However, it does not work well with gdm3: external monitors don’t work because the nvidia driver crashes; the log files indicate the x-session is providing bad data, which must be something to do with gdm3. I have never got gdm3 working with Optimus when mode setting is turned on. You can at least log in: the intel driver works.
This bug with gdm3, nvidia and mode setting in x.org sessions is upstream and long-standing, and no one seems to be working on it. So ubuntu users with Optimus should stick with lightdm
Alberto plans to backport the changes to 18.04, at which point the workaround at https://github.com/timrichardson/Prime-Ubuntu-18.04 will be redundant