How long do you have? And i suppose, how often will you be doing this sort of thing? There’s huge amounts of riding without getting out of the borders but it’s kind of a shame to “do” scotland and not get up further. Oh and, what sort of riding- trail centres, big natural mountain rides?
Fort William (Nevis Range) itself is imo a day’s riding. The trailcentre stuff is alright but nothing special so I reckon go for the uplift or don’t go at all. The downhill and uplifted red are ace if you’re into that, and kind of iconic to boot so that’s a nice thing to have in your plans. It’s a long drive from the borders so you’d want at least another day’s riding in there, Kinlochleven perhaps (amazing rocky trailbike stuff there and quite easy to navigate) or a big natural mountain ride.
You could easily spend a week riding in the tweed valley alone, though that depends what you want to ride really…