can I use Spacedesk to simulate a virtual monitor for example with 8K resolution, in order to make 8K screenshots of a web page or of an application window?
Yes you can simulate an 8k virtual monitor with spacedesk.
Just enable the videowall mode, then create a video wall by assigning a client to a wall.
The default dimension of a spacedesk video wall is 4K (3840×2160) but you can modify it and set an 8K resolution.