There are things that do not work on Wayland yet

Depends on your compositor. GNOME is probably not going to support your use case, but you could get `river` to do it.

> My use case is a script that launches a zillion of terminal emulator windows, some running a specific application (e.g. screen /dev/serial/by-id/... or ssh), carefully laid-out on my multi-monitor setup.

First, I would use `tmux` and its layout controls (still running `screen` because, yes, it is very good at talking to serial devices). If you need to multiplex commands, there is the `synchronize-panes` option to send input to all panes in a window. But if you want to have top-level windows, here's how I would do it in XMonad:

- set the layout for the workspace to Grid (or anything that is suitable for you)
- launch all the windows while it is active (or have rules that put them on that workspace)

With `river`, I would do it largely the same (though s/workspace/tag/). However, I'll note that I am still in the process of migrating to river, so this is just based on my limited understanding of its capabilities, not from experience.