Isolating Sunshine run into another user/session

[skryvel]

This is a general question, but given how complex it is and that I imagine it's relatable to the audience here, I figured I'd cross post.

https://superuser.com/questions/1840849/how-can-i-programatically-start-a-new-session-as-another-user-in-the-same-seat

It's essentially the following, with more details:

Within a running session (KDE6, wayland) that is locked, from a daemon (KDE Connect), I want to start a session (Gamescope/Steam) as another user and run a streaming software (Sunshine), which needs to have GPU access/lease/something and be able to capture (KMS?) the screen. I do not want to just run Steam as another user within the same desktop session.

The KDE connect user has been started through SDDM quite normally. There is only a single seat. The newly logged in user will take over the seat. The objective is to achieve some isolation, since Sunshine is essentially a remote command tool, and so I don't want to expose my normal user. Containerization and so on is tricky because this is about gaming and sharing a bunch of things and this level of isolation is enough for me, as long as I can lock the normal user. The GPU driver is amdgpu. The target user is a regular one and has a shell (for now) and has succesfully logged in via SSDM/KDE once.