JAWS 2020 in Firefox and Chrome and NVDA 2019.3.1 in Firefox (Windows 10), and VoiceOver in Safari on Mac OS Mojave 10.14.6 all announce the button text in all examples, followed by the complete aria-describedby text after a pause. However...
NVDA 2019.3.1 in Chrome 80 only reads aria-describedby content that is not contained within <strong> and >em> elements. For buttons 2 and 3, it simply reads "This is" and omits the remaining text. It does not have this problem with the <span> element in the fourth example.