FWIW I have another suggestion for your consideration. Since the Fusion drive has come apart, rather than trying to put it back together, you might leave the HD and SSD as separate drives. Previously you mentioned the SSD is 200GB and that is more than ample space for a boot drive. (MacBook Airs come with 128GB SSDs).
Do an Option+Command+R install of MacOS on the SSD (that will be the latest version of MacOS compatible with your computer). Format the HD either as APFS or MacOS extended (MacOS extended is a bit faster on rotating rust but APFS is so much more flexible) then follow
these instructions to move the User folder from the SSD to the HD. (Ignore the part about it having to be formatted MacOS Extended this will work just fine with either APFS or MacOS Extended).
By having the boot system on the SSD you will get optimum system performance at the cost of being a bit slower with large file access, but you may not even notice the difference. I did essentially the same thing with the Fusion drive in an old Mac mini several years ago and that Mac mini still works for me today as a file and printer server. I now have three separate APFS volumes on the HD sharing the same 1TB HD’s capacity. (In my opinion, APFS volumes beat the heck out of partitions for usefulness and flexibility,)
It won’t put the Fusion drive back together, but it will get you back up and running again. By-the-way, you could even install a bootable copy of MacOS on one volume of the HD to have around in case of a future emergency.
