Remember the plan was to assimilate Earth in the present. Using the sphere and time travel was IMO something the Borg came up with on the fly, after their massive cube was destroyed.
The big plot hole of FC is that the Borg could've stayed in the DQ and used a time sphere to travel back in time there and THEN assimilated Earth in the past. It's a solid plan, and from FC we see that it can work. Why the Borg never tried to do it again all throughout VOY is beyond me.
I've always wondered... was FC before or after Voyager got back home??
And as to why the Borg didn't try again, it failed the first time, and the Borg don't think like "if at first you don't succeed, try again." To them, if it didn't work the first time, it will never work.