43mm rake fork? What's the HT angle? It's going to have to be really steep to get the trail where you want it for aggressive handling.

I recently did a custom frame. HT angle is slack (I'm short) at 71.5. But the fork rake of 50mm put the front wheel out enough to have short trail. Additionally, the BB is a bit lower than normal to drop CoG. Handling is exactly what I wanted.

Regarding the seatpost, often times to locate the BB in the right spot and the chain stays at an appropriate length for application, seatback seatposts are required.