Nice work! Also, you should update the code, since #19957 has been merged.

A few things:

Can you add this so it will be possible to change the separator and indent.

        $leaf = new TextPropertyItem(
            'separator',
            __('Columns separated with:'),
        );
        $generalOptions->addProperty($leaf);
        $leaf = new TextPropertyItem(
            'indent',
            __('Indent'),
        );
        $generalOptions->addProperty($leaf);

You should also make changes in src\Config\Settings\Export.php so the values can be seen when exporting and also be able to use them from config file

$cfg['Export']['toon_separator'] = '|';
$cfg['Export']['toon_indent'] = 4;

Also, can you add a small text to documentation import_export.rst - #export?

Thank you!

@faissaloux I left some comments.

1. Add support for tab as delimiter - https://toonformat.dev/playground.html
2. Renamed from Toon to TOON