I didn’t water-proof outdoor cushions but I did scotchguard my indoor sofa a few years back. Right afterwards I read about how the chemical in scotchguard is really bad for you. I haven’t done that again. ☹️
However, your idea for recovering with Sunbrella fabric is a good one. Our outdoor furniture and umbrellas are covered in Sunbrella fabric and it has held up so well. We leave the cushions out for 5 months in the summer in full sun and don’t cover it. No fading that I can see, very durable and it dries off very quickly.
" I object to violence because when it appears to do good, the good is only temporary; the evil it does is permanent." Mahatma Gandhi
"This is the ultimate weakness of violence: It multiplies evil and violence in the universe. It doesn't solve any problems." Martin Luther King, Jr.