I was able to use my fairly complex REPL script to launch Reveal with customised UI using the custom commands. I added the whole script to a separate file (https://github.com/sauvala/dotfiles/blob/main/calva/launch_reveal.clj) so it doesn’t take space in the settings and then load it:

"calva.customREPLCommandSnippets": [
    {
        "name": "Load Reveal",
        "repl": "clj",
        "snippet": "(load-file \"/Users/janne/.config/calva/launch_reveal.clj\")"
    },
]