Hi Leigh
thanks for your words.
To answer your question, you can download the plugin "decimation Master" from the plugins section from Pixologic site.
This plugin allows you to decimate your model from, for example, 5,000,000 polygons to, for example, 50,000 polygons without a visible loss of detail.
My models were originally composed of a global number of polygons about 11,000,000, with the decimation Master I took them to about 450,000 polygons without having a particular loss of detail.
Also download the manual, which will teach you how to use it.
greetings and have fun zbrushing