Finally solved it. As I could find out here Windows 10 DISM error: 0x800f081f the ISO created by the Windows 10 Media Creation tool does not work with DISM. I downloaded one from Techbench and managed to use DISM with that one.

I do not know whether it is the fact that the ISO created by the Tool has an ESD file instead of a WIM one, but changing the ISO to the Techbench one (which uses a WIM file instead) and using the adapted command DISM /Online /Cleanup-Image /RestoreHealth /Source:WIM:G:\sources\install.wim:1 did the trick

I cannot accept my own answer for 2 days, so will have to wait to mark this as answered

Finally solved it. As I could find out here Windows 10 DISM error: 0x800f081f the ISO created by the Windows 10 Media Creation tool does not work with DISM. I downloaded one from Techbench and managed to use DISM with that one.

I do not know whether it is the fact that the ISO created by the Tool has an ESD file instead of a WIM one, but changing the ISO to the Techbench one (which uses a WIM file instead) and using the adapted command DISM /Online /Cleanup-Image /RestoreHealth /Source:WIM:G:\sources\install.wim:1 did the trick

I cannot accept my own answer for 2 days, so will have to wait to mark this as answered