For the OP,
I think you ordered the right thing, and the wait is worth it. See the 2013 DIY SSD thread for a little more info on my recent experiences with 2012 and 2013 iMacs with the different drive configurations and upgrades I did,
https://forums.macrumors.com/threads/1647795/
Basically, the HDD-only is way too slow in my opinion, doesn't keep up with the great performance of the rest of the 2012 or 2013 iMac. The SSD is a seamless experience, but obviously limited in terms of the size of a single drive vs cost.
The Fusion is nearly-SSD. Depending on how you use your machine, you might notice "no difference from SSD" to an occasional awareness of when the Fusion goes to the HDD instead of the SSD, which is very minor, very normal. To have high speed and one huge drive, the Fusion sounds like the option to meet your goals.
So, good job in your choice, I think you did the right thing in ordering the Fusion, despite the wait.
Just for reference, to get SSD-like speed in a higher capacity, i.e. >2TB, the next step is perhaps a Thunderbolt enclosure with multiple SSD's in a RAID configuration... which can get very expensive very quickly.