QGIS - print composer legend default settings

Question

In the QGIS user guide, it says

The legend will be updated automatically if checkbox Auto-update is checked. When Auto-update is unchecked this will give you more control over the legend items.

Can I change any settings so that Auto-update is not checked when the legend is created? And can I build the legend from scratch by default, rather than dismantle it by having to remove unwanted layers (many of which may be invisible on the map canvas).

The reason is: when I have a layout that includes raster layers with bands (in my case Ordnance Survey tif), the legend takes a long time to load when first created then unchecking the box takes ages, then deleting the unwanted raster layers... If I have a lot of these layers, it can take over 5 minutes just to load the legend, so I can be looking at nearly 10 minutes just to get to the point that I can edit the legend.

I'm currently using QGIS 3.8.2

The similar question has not been answered in a way that solves the question I have asked, although it is a loose work around: QGIS- how to prevent layers from appearing in legend on composer

Answer 1

Very similar answer to the previous question you linked, but it's the only way I can think of to achieve this:

1. Create a new layout in a blank QGIS project. Add a legend with the Auto-update box unchecked and save this layout as a template.
2. In the layout view of your main project which has your raster layers, select 'Layout' > 'Add items from template...' and navigate to the template you made in point 1. This will add the legend without altering any other objects you've already set up in your layout (map, north arrow etc...)