QGIS 3: Categorised symbology with value driven alpha transparency

I'm going to answer this one.

Open the symbology pane. Use categorise and set a ramp. Classify. Select all classifications, click the down arrow next to symbols up the top. Select Configure Symbol. Click the word "simple fill" in the top panel. Click the menu next to fill_colour, about 3/4 the way down the menu click edit.

Make it look like the following, with a simple line and simple fill. You can do this with all categories by selecting all categories, then adding a simple line and clicking apply/ok to close the dialogue.

Then, you want the lines colour to always match the simple fill. Do this by using the following code snippet in the place indicated by the following picture.

color_rgba(color_part(@symbol_color, 'red'),color_part(@symbol_color, 'green'),color_part(@symbol_color, 'blue'),color_part(@symbol_color, 'alpha'))

Then, when you wish to modify colours / transparency, you do it through the main window, per the below, with the "Fill" symbol selected.

There is a bug where the line will display its original colour, but it will render the colour of the fill.

Opactity can be changed programtically based on the value of a field, and the fill and the line will respect the fill.

This method allows you to eliminate the presence of visible lines, white lines, lines not appearing, etc. It allows you to use a colour ramp to classify the colours, and have the line colour follow the main fill colour. It allows you to set the alpha channel opacity / transparency based on the value of a field and have all the symbology change colour and opacity together. It can be applied to the entire symbology set at once.