An AVR only has one 16 bit timer with two compare channels, so either you can only use two channels at full resolution or you have to waste lots of cycles sampling it.
The other hassle is having loads of wires running from the rx to the micro.
The receiver on the FlySky 9x uses SPI to send data from the radio chip to the chip that actually generates the timings on each output channel.
If we tap off the SPI lines we can listen directly to the serial data the radio is spewing out... Luckily its super easy to understand!
Step 1: Cables
I'd recommend 80 way ATA hard drive cable (the thin stuff, not like this)
Step 2: Cutting
Step 3: Power
The edge one is +ve, the middle is -ve.
Step 4: Data
We're soldering onto the MOSI and SCK pins shown in the picture.