How much driving do you want to do? Lots of places on the map look pretty close to each other ATCF, but in reality are a fair old drive.
Frinstance, Skye looks as if it’s ‘just past fort William’, in reality it’s a fair old run either way (and the riding’s no worth it IMHO)
I suggested the Tweed valley based on this reasoning, bang for buck, I can’t think of another area in the country that has as many trails, natural, hand cut or trail centre type.
Aviemore and Laggan is a good option to base yourself, as is Fort william, easily 5 days great riding around those areas too.