Stop debugging the issue.
A couple of things might come together, one of it is that the currently available VBoxCLient 6.1.6 in nixpkgs is buggy and simply can’t do the resizing anymore.
Some more community effort is also trying to create a workaround by using another graphicsdriver which hasn’t been merged the last time I checked. That one also comes with reconfiguring your machines graphicsadapter as far as I understood (VMSVGA instead of VBOXVGA or something).
For more infor take a look at Does anybody have working automatic-resizing in VirtualBox?