How to correct the GPT of this non-booting fusion drive iMac?

Question

Question renamed from: « Diagnosing what's going wrong with the fusion drive of an iMac 27'' 2019 A2115 EMC 3194 » after the origin of the problem could be located.

Having a non-booting iMac using fusion drive, with a 32GB SSD and 1TB HDD, I would like attempt restoring it without loosing the data. I can still start the iMac in recovery mode and issue commands through the terminal.

I did two external clones of both the SSD and HDD. There were no errors during the cloning. Furthermore the sha1 /dev/... command confirmed identical SHA1 hashes for the original disks and their clones. So, I can restore the situation by cloning in the other direction if necessary.

-bash-3.2# diskutil list
/dev/disk0 (internal):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                         28.0 GB    disk0
   1:                        EFI EFI                     314.6 MB   disk0s1
   2:                 Apple_APFS Container disk3         27.7 GB    disk0s2
/dev/disk1 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk1
   1:                        EFI EFI                     209.7 MB   disk1s1
   2:                 Apple_APFS Container disk3         1000.0 GB  disk1s2
/dev/disk2 (disk image):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        +2.1 GB     disk2
   1:                  Apple_HFS macOS Base System       2.0 GB     disk2s1
/dev/disk3 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +1.0 TB     disk3
                                 Physical Stores disk0s2, disk1s2
   1:                APFS Volume Preboot                 185.9 MB   disk3s2
   2:                APFS Volume Recovery                510.6 MB   disk3s3
   3:                APFS Volume VM                      2.1 GB     disk3s4
(...)

diskutil verifyDisk /dev/disk1 reveals that the problem comes from the EFI partition on the mechanical hard drive:
This first problem could be corrected. See "Edit" section below.

-bash-3.2# diskutil verifyDisk /dev/disk1
Started partition map verification on disk1
Checking prerequisites
Checking the partition list
Checking the partition map size
Checking for an EFI system partition
Checking the EFI system partition's size
Checking the EFI system partition's file system
Problems were found with the partition map which might prevent booting
Error: -69766: The partition map needs to be repaired because there's a problem with the EFI system partition's file system
Underlying error: 8

I assume that I should destroy the GPT of the HDD and create it again, or at least modify it. In David Anderson's answer here, one can see how to remove GPT entries and create ones. But I am not sure if I should create again the GPT of the HDD with the same values or if some have to be corrected.

Here's the output of gpt -r show disk1 for the HDD:

gpt -r show disk1
       start        size  index  contents
           0           1         PMBR
           1           1         Pri GPT header
           2          32         Pri GPT table
          34           6         
          40      409600      1  GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B
      409640  1953115488      2  GPT part - 7C3457EF-0000-11AA-AA11-00306543ECAC
  1953525128           7         
  1953525135          32         Sec GPT table
  1953525167           1         Sec GPT header

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Edit

I deleted the EFI partition and created it again. diskutil info disk1s1 (and made screenshot of partition info) diskutil umountDisk disk1 (unecessary, but just to be sure)

gpt remove -i 1 /dev/disk1
gpt add -i 1 -b 40 -s 409600 -t C12A7328-F81F-11D2-BA4B-00A0C93EC93B /dev/disk1
gpt -r show disk1  (verify gpt table)
diskutil info disk1s1  (verify that the partition is still FAT32 and EFI
diskutil verifyDisk /dev/disk1  (verify partition map on disk1)

The partition map is now OK for disk1.

The verification fails on the verification of the volume, on disk0s2:

error: fusion_mid_tree: btn: invalid o_oid (0x0) 
Fusion data structures are invalid 
The volume /dev/disk0s2 could not be verified completely 
Storage system check exit code is 8 
Error: -69716: Storage system verify or repair failed
Underlying error: 8

When calling diskutil resetFusion (the iMac seems using Mojave), only the rotational disk (HDD) is detected:

diskutil resetFusion
Internally-located hardware disk device known to the currently-running macOS:
Rotational  (disk1)
Volumes exported by partitions or storage systems hosted on the above device:
Your computer must have exactly 1 solid-state and 1 rotational disk drive.

In summary, the problem seems coming from the fact that although the SSD is listed by diskutil, it is is ignored for the fusion drive. So, the question is how to repair the fusion data structures?

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