Nested grids

It's technically possible, but I wouldn't advise it. It makes the widget really difficult to deal with and generally you can achieve what you want by just doing a 12 col and properly assigning classes.

Anything beyond that probably doesn't belong in WYSIWYG to be honest and should be more separated with Layout options (i.e. Layout Builder) or separated fields.

Thanks for the response. WIth layout builder indeed there's an option, just that you are restricted to 2 levels (1 in layout builder and 1 in wysiwyg in grid). It's an edge case though that you'd need more nestings.
For those looking for an option, I've found the option to use embedded components https://www.drupal.org/project/embedded_content → with a custom component to use form type text_processed in which you can add the grid + these patches to allow nested modals + https://www.drupal.org/project/embedded_content/issues/3401138 🐛 Embedding media inside an Embedded Content plugin Active + https://www.drupal.org/node/2741877 →