Sounds greedy, but all at once sounds feasible.
Wish to:
Change default material.
Stop the announcement scene (features etc)
Most importantly stop lightbox
Know about disable in preferences, new startup document.
Nothing works for lightbox, would be willing to uninstall if possible.
Thank you
Lightbox is the easiest. Preferences > Lightbox > Open At Launch (disable, then store configuration). There does appear to be some sort of bug where the function gets re-enabled as soon as you store the config, but as long as you store the config while it is disabled, it should not open the next time you start zbrush. This works for me at any rate, and if it isn’t working for you theres probably something else going on with your Zbrush installation.
The news viewer (is that what it’s called?) can be mostly disabled by clicking the gear icon in the top right of the viewer window, and setting it to open only when there’s is something new to report. This happens very infrequently, so you’ll almost never see it, and in the event you do it will probably be something it would be good to be notified about (like an update). In fact, I couldnt even figure out how to switch it back on to research this post 
[Edit–It’s in Zplugins > Misc Utilities > Home Page]
The default material I’ll let Marcus handle. I notice when using my custom UIs it saves the material that was active when I saved it, but I’m not sure that’s working correctly.
Bless you.
Was saving startup doc, forgot configuration.
Thank you