I am aware of the
cycle_workspaces setting but I want to have two shortcuts:
cycles over the windows of the current workspace
cycles over the windows of all workspaces
This is not exactly what you are looking for, but may help.
Sve this code to a file (don't forget chmod +x) and map a hotkey to execute this script, for example Win+Tab. This script will toggle the /general/cycle_workspaces setting so that the next time you use Alt+Tab it will cycle only the current workspace or all workspaces.
You can customize what is included in Alt + Tab using
Maybe in addition to Alt + Tab you want to use a task-switcher such as