Thanks @fomightez and @phaustin.
I tried to reinstall Jupyter with/without the desktop app, after cleaning all the Jupyter environment variables I could find (inside Library and .jupyter in particular) and now nothing works at all (cannot run any code without “cannot generate requests” or “kernel cannot be started” or complaints about the ipykernel, which I have installed in the environment). It’s too much of a mess at this point: I’m going to destroy the environment and try again one last time. UPDATE: Jupyter is working! JupyterLab desktop is working! It’s able to find my environment and use the dedicated kernel!
background: I probably didn’t need Jupytext after all, since I do not plan to edit notebooks: my use case is to generate notebooks from markdown. My interest in the JupyterLab desktop app was because I do not want the notebooks to open in browsers. I had been using nteract up to this point, which is rather limiting, and the desktop app looked very promising. I’ll report back if I have any success in repairing jupyter. Thanks for your help!