Darkcyan, I think the geometry on the Fortitude suits you for better than the Niner you had before. The slacker head angle, longer wheelbase and compliancy of the steel tubing seem to give you the ride you’re looking for. Bigger volume tyres make a huge difference too, allowing you to run lower pressures safely and reap the benefits. You look noticeably more confident as a rider when following you on the Fortitude compared with the Niner, and there’s no doubting you’re quicker on the Fortitude even from the first ride out.
As TimP said, it would probably be quicker and more controlled with suspension forks, but only really necessary if you were going to ride it outside SE England. The local trails are super smooth and fine on a rigid bike for the most part.
Forget the Niner and stick with the Fortitude on the evidence you’ve given here and the experience you had at BigDog. The only changes I’d make would be to swap to carbon bars and seatpost to dampen the trail chatter and drop some weight, and invest in some lighter wheels with decent width rims and you’ll have a bike for life I reckon. Genesis seem to have recognised this with the latest incarnation of the Fortitude specced with much wider rims and the option of running 29+ tyres.
TimP, I love it that your quickest lap at BigDog was on some borrowed wagon wheels 😉