Write topics all to one line

Set the default line width to a high value.

Select a topic > right click > Format Topic > Size and Margins > Preferred Width > Apply

Click Map Theme (bottom right) > Save in Default Theme for this map

All topics of this type will have this setting on this map. Do the same for other topic types or levels if required.

Save as a Template (after deleting unwanted content), so new maps can be started with these settings.
Save as a Style to apply subsequently to existing maps.

Just found this really neat tip. Thank you Andrew!