InstallerNG was crapping out on me on basically every install, so I installed Installer-43_3.lha from Aminet, which let me actually install things without crashing
The MUI version I installed was mui38usr.lha from Aminet
After which the A600GS wouldn't boot in to Amibench, so I updated the A600GS firmware and restored from defaults, which made it work again.
IBrowse installation finishes nicely (if I assign MUI: to where I installed MUI previously) but it won't start, it gives me an error on one of the MUI modules/classes/something