Thanks to the comment above I discovered Games on Whales and just read up on the possibilities. Isn’t that exactly your usecase? It seems you can stream to multiple underpowered systems provided you have network, RAM, GPU and enough cores. It even streams to Apple TV, Chromecast etc.

No, I mean to have multiple concurrent sessions, e.g. to have friends over on laptops playing games from a centralized device

That's exactly what Game on Whales offers

It's multi seat.

Yes, because it uses containers. It’s not using VMs which would probably be required e.g. for many windows games

Not sure if LXCs would help in that regard as I haven't used it for gaming but you can share a GPU between LXC guests and the host. I have an unprivileged LXC running in Proxmox that uses the GPU for transcoding video.

AFAIK there is no way to share a GPU between a host and multiple VMs without commercial hardware

Thats the thing, LXC isn't a VM, its a container, so they all share the same kernel. I think its worth a shot, so I may test this.

FWIW Incus is really good here. I don't quite have it fully working for headless gaming containers sharing a single GPU, but I'm very confident it will work at this point.

It's possible to run vgpu with modded drivers up to, I think, RTX 2xxx series.

intels gpu supports vgpu with no extra licence