Windows 10 dual boot with High Sierra on the same SSD of an Mid-2011 iMac

I have installed Windows 10 (in UEFI mode – no bios emulation) first on an empty SSD of an iMac (mid-2011). Once I was done I attempted to perform a usb-install of High Sierra (I have a created an HFS+ partition around 180GB or so at the end of the SSD). However the MacOS X installer for High Sierra complains that the disk in question cannot be a startup disk. Partitioning info looks like so:

-bash-3.2# diskutil list disk0  /dev/disk0 (internal, physical):     #:                       TYPE NAME                    SIZE       IDENTIFIER    0:      GUID_partition_scheme                        *512.1 GB   disk0    1:           Windows Recovery                         523.2 MB   disk0s1    2:                        EFI NO NAME                 104.9 MB   disk0s2    3:         Microsoft Reserved                         16.8 MB    disk0s3    4:       Microsoft Basic Data BOOTCAMP                322.1 GB   disk0s4    5:                  Apple_HFS Windows Shell EFI       67.1 MB    disk0s5    6:                  Apple_HFS MacOS                   188.2 GB   disk0s6  -bash-3.2# gpt -r show disk0        start        size  index  contents            0           1         PMBR            1           1         Pri GPT header            2          32         Pri GPT table           34        2014                  2048     1021952      1  GPT part - DE94BBA4-06D1-4D40-A16A-BFD50179D6AC      1024000      204800      2  GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B      1228800       32768      3  GPT part - E3C9E316-0B5C-4DB8-817D-F92DF00215AE      1261568   629114880      4  GPT part - EBD0A0A2-B9E5-4433-87C0-68B6B72699C7    630376448      131072      5  GPT part - 48465300-0000-11AA-AA11-00306543ECAC    630507520   367659663      6  GPT part - 48465300-0000-11AA-AA11-00306543ECAC    998167183     2048000            1000215183          32         Sec GPT table   1000215215           1         Sec GPT header 

The only thing I can imagine being wrong is that the EFI partition is either too small or is not being first on the disk. If that’s the case is there a way to salvage the situation without reformatting everything?