Idea

Primary laptop will run DE with 1 screen (its own).
Second laptop is to be used as a secondary (extended) monitor for primary laptop.
Both laptops will run Linux.

Tested solutions and their issues

krfb-virtualmonitor:

Creates virtual display and runs a VNC server, exactly as desired.
Issue: Slow as hell. FPS become SPF even on 0.3ms 1Gbps point-to-point connection.

KDE Connect virtual display

Creates virtual display and runs an RDP server, exactly as desired.
Issue(s): Really needs that high quality connection to run reasonably well. That isn’t much of a problem, I can use a cable. The biggest problem is, no matter the settings, both with Remmina and KRDC the mouse pointer does not show up. Silly issue, but yeah.
Also, the Debian 13 KDE Connect package is too outdated to work with this, but I was able to use it in KDE Neon and Arch.

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So, basically what I want exists, just with a major bug. Not seeing the mouse pointer is quite a problem.

Also, at first I hoped to use the already existing “network”, my phone’s hotspot. I can get 300Mbps and 3ms between phone and client. But between 2 clients? 400ms and with ping I see 1/3 of packets being duplicated.