Grouping vector file column attributes under another column's attribute as a hierarchy group

I would like to output attributes in a vector file I have for Land use. There are three categories that are in hierarchical order. The columns represented in the attributes table are:

PRIMARY_20 > SECONDARY_ > TERTIARY_2

I would like to display the TERTIARY_2 column attributes but group them under their PRIMARY_20 category so it looks like this:

PRIMARY_20 (no symbology, just group heading)

(grouped below PRIMARY_20) TERTIARY_2 (with symbology displayed in the map layers panel and in the map legend (layout))

I have tried the method outlined here (I added the SECONDARY column just as a test): https://gis.stackexchange.com/questions/34428/dissolve-column-based-on-attribute-in-another-column[![enter image description here]2]2

and my own method trying to concatenate (see figure attached).

I can get the column attributes to join but cannot space them ( I have tried ‘ ‘ between concatenate) and tried to insert a new line before TERTIARY_2 (using ‘n’ function) to no avail.

I have attached figures to try and explain visually.