Buy and install your MegaEsim a day or two before you leave home. You need WiFi to install it, and your own home WiFi is far safer and faster than the crowded signal at Nadi International Airport. The whole setup takes about five minutes. You scan one QR code and the plan loads onto your phone.
Installing does not start the clock. Your days only begin counting once you arrive in Fiji and the eSIM connects to Vodafone. So you can get everything ready early and not waste a single day. Just leave the plan switched off until you land.