GParted, a partition manager, ships on the Live ISO. It is also installed OOTB on Solus installations. We have thorough documentation on creation of ESPs and always recommend you do free space creation via Windows 10 first, since it can better shuffle its own files than GParted can.
We can only install when there is free space to do so as part of the dual-boot scenario. We also literally state in the manual partitioning section to use Gparted, see here.