£800 is cheap for a Ti frame but that’s not to say it’s poor quality. The new Ti456s are made by Van Nicolas in their Chinese factory whereas before they were made by Lynskey in the US. That’s why they can now sell them for £800.
They are a bit of a design classic and the original Lynskey one I owned (albeit briefly) was lovely. But then so were the Cotic Soul and the Dialed Alpine I also owned (for a similar length of time).
The best hard tail I’ve ever owned, and the one I should never have got rid of, was the Ragley Ti. That was an amazing bike; stiff where you needed it, compliant elsewhere with excellent geometry for really aggressive riding.
The Soul though is/was a close second and is probably the closest thing to the Ti456 in terms of character without being designed to take a 160mm fork. If you want the longer fork then the BFe is identical to the Soul in terms of geometry but can take the 160mm fork. The Soul is a lovely bike rewarding precision and finesse with a fabulous ride quality. I haven’t ridden a BFe but I imagine it will handle like a Soul but reward a more direct, manhandling style of riding.
The Dialed Alpine was also a hoot. There is a new one coming though so best to wait for that as the original has the same limitations you’ve identified with the Stanton Slackline.
One other bike to consider though is the Slackline Ti, which will have a 44mm headtube and 30.9mm seat tube, but you’re going to spend a lot more than £800!