Also built in to Windows these days is the Emoji (Soft) Keyboard which you can pull up with Win+. or Win+; (whichever you prefer). Since a few Windows feature updates back it even has a comprehensive math symbols section (under the Omega tab). It just needs a few more shortcuts under the math tab to make it really powerful. For instance, if you keep typing after you bring up the Emoji Keyboard it will search through Emoji and then replace your search word with your final selection, but it currently doesn't let you type search words or shortcuts for math symbols when you switch to the Omega tab.