@gidi there is a ticket to allow the preview of block themes, but it looks inactive.
https://github.com/WordPress/gutenberg/issues/37201
I remember there are plugins to switch themes based on the user, so admins can make changes before switching to a theme for the public, but since the templates are essentially content, they don’t seem to work without affecting the active theme.
I suggest hiding blocks or areas you don’t need or need to be updated, using a plugin like Block Visibility, or setting a Staging site to make changes in a safe environment.
https://blockvisibility.mystagingwebsite.com/knowledge-base/how-to-use-the-hide-block-control/