"The AS5145B is the equivalent of an optical encoder with a 1024 count disc, meaning it will output 1024 pulses per channel per revolution. In 4x decoding mode, this will yield 4096 ticks per revolution."
In that context, "pulses" seems to mean "cycles" (whereas
US Signal uses "pulses" to mean "edge transitions").
Assuming that is correct, there are 1024 rising and 1024 falling edges per rev on each channel.
Let's do the math. There are 2*1024 = 2048 edge transitions per rev on each channel. So the edges are 1/2048 of a rotation apart,
assuming the best case of perfect symmetry. So the max speed the FPGA can handle for a single channel of that encoder before it starts to get weird is (1/2048)/6.525e-6 = 74.8 rotations per second = 4490 RPM.
So that will not work for your desired 6000 RPM. Use a different encoder with fewer cycles per rev.