I am guessing that the Lattice ICE40LP FPGA would help a lot with data processing.
I doubt that FGPA will do anything else than interfacing between the AT86RF215 and the Raspberry Pi. It is pretty small in terms of the number of gates - but also cheap (around $3 at quantity).
You have ~20 GFlops videocore + 4 ARM cores (around Core2 performance)
Cortex SMALL (A57) cores are nowhere near core2 performance. Comparable to first generation Atom possibly. In-order simple cores.
pi4 is around Core 2 Duo E4300 single thread, maybe even faster than E6700 in multicore. This is ~ N3350/Z8350 performance.