brent Not really )))
When you take off the boot flag from Win partition and assign BOOT and ESP to future Solus efi partition, then the Solus installer recognizes where to set boot. During installation you can not choose boot partition - only root, where all OS fils will be.
But as you had taken off the flag from Win partition, Win won't boot, if you forget to put it back after Solus installation.
And here happens the following: when you assign in GParted the boot flag back to Win, the boot flag from Solus EFI disappeares. Only ESP stays.
If you try to put boot flag again to Solus EFI, Win will have no boot flag and won't boot.
But it is OK!
You have already made the Solus EFI partition and together with Win boot flagged back you are supposed to have dual boot.
It was really much more simple than I was trying to do before, and the issue is solved. By the time being ))
Another man here got more serious problem - he was already using Solus , but the boot broke, and he can not reach the files on Solus partition, to load into installed Solus, And that's why he can nnt just make the new installation.
I will follow that issue, because the problem seems to be not rare. So, it is important to know the method.