The best people to answer the Visa D questions are probably the Austrian Embassy in London. As far as I am aware, it comes in two versions. One is a tourist Visa which does not allow you to work (including home working) and for the work Visa you will need an employer/sponsor: https://www.bmeia.gv.at/en/austrian-embassy-london/travelling-to-austria/entry-requirements-visa-and-immigration/visa-d/