eGPUs were always iffy but Optimus does work on some laptops that already have an nVidia/Intel hybrid setup. At least on Windows.

You need a Thunderbolt/PCI-E enclosure and an nVidia card, then you can basically replace the internal dedicated card with your eGPU and it works in hybrid mode as normal.

It's only DIY stuff, but it's using nVidia's own Optimus drivers, so it's probably stable.

Sadly, System76's implementation doesn't work with Windows (likely due to them using Coreboot) but it does work with Linux. I actually find that impressive, must've taken a lot of work to make it... work.