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