The Fusion Drive is a combination of a hard drive and an SSD. It uses SSD to load your frequently accessed files on the Mac. Although is faster than the traditional hard drive (HDD), it still cannot outperform the data loading speed and stability of a pure SSD.
If you seek high storage capacity, an external hard drive can be replenishment with an SSD-equipped Mac. It is unnecessary to choose a Mac machine that is built with a Fusion Drive for larger storage capacity.
By the way, have you recovered data from your dead iMac? To recover data from an unbootable iMac with a Fusion Drive, you have to use a professional data recovery tool to help you. iBoysoft Mac Recovery Mode can enable you to rescue files from an unbootable iMac without creating a bootable drive.