Fusion drive on iMac model 17,1, Mojave 10.14 - APFS or HFS+ format for HDD?

I [have read](https://iboysoft.com/wiki/apfs.html#can-you-use-apfs-on-hdd?) that APFS was *optimized* for SSDs, but that it's not an optimal choice[ for HDDs. However, I just got my iMac (Model 17,1; OS X Mojave, 10.14) back from repair, and both drives are formatted as APFS. I checked a [diskutil output](https://apple.stackexchange.com/questions/446755/imac-mojave-suddenly-became-extremely-slow-on-login) made before the repair work was done (i.e. as it came from the factory), and it seems that both drives were APFS when my iMac was originally built. Is the APFS format on the HDD in a Fusion Drive a necessity (perhaps due the the need for the SSD using APFS)? If not, could performance/stability be enhanced by using HFS+ on the HDD? RE: "Has it been reformatted as a Fusion Drive?" The diskutil list output:

necromac:~ seamus $ diskutil list
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.3 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                 Apple_APFS Container disk2         500.1 GB   disk0s2

/dev/disk1 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *3.0 TB     disk1
   1:                        EFI EFI                     209.7 MB   disk1s1
   2:                 Apple_APFS Container disk2         3.0 TB     disk1s2

/dev/disk2 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +3.5 TB     disk2
                                 Physical Stores disk0s2, disk1s2
   1:                APFS Volume Macintosh HD            139.3 GB   disk2s1
   2:                APFS Volume Preboot                 52.0 MB    disk2s2
   3:                APFS Volume Recovery                510.6 MB   disk2s3
   4:                APFS Volume VM                      2.1 GB     disk2s4

... and diskutil screenshots: | Screenshot | Screenshot | | -------- | -------------- | ||| || empty|