Can't install Mojave from App Store downloaded installer

I'm trying to upgrade my wife's late 2013 iMac 27" from High Sierra to Mojave. I did the same upgrade on my MacBook Air, which is slightly older than my wife's iMac, a couple of months ago and it went without a hitch. I have downloaded the Mojave installer from the App store, and deleted it, a number of times over the last day or two. It downloads and opens without any error messages. I can accept terms and conditions, select my hard drive and enter an admin password, but then it just hangs, and I'm left with the revolving beach ball. I can't quit the installer, so until I've force-started the computer. I've run disk first aid, reset VRAM and SMC, and tried quitting all opens apps before running the installer. My wife's iMac has plenty of RAM and free disk space. Apple chat support recommends creating a bootable Mojave install USB stick – but I'm not really convinced it would do the trick. I'm tearing my hair out, surely there's an easier way? Any advice would be most welcome.