I am exactly doubting your suggestion of tax paid donations. I don't think this will happen, unless we actually come together and try to actually enforce this on the political level in various countries.
After all, open source software is an essential and critical foundation since many decades - but I'm not sure, whether there is any government that has made a pledge to donate a certain amount of money per year into the development and funding of such general purpose software. (Maybe I'm wrong though.)
Before the fediverse can get any public funding, we need to make some political efforts. the UN is the largest such institution - and it took all the fiasco with the 2 world war to get many countries pledge to donate to it every year...