The latest prototype is in, and it works! The board has 2 2-channel op-amps now, and 4 digital switches, which allow for 2 gains for each channel independently: +-4 V and +-400 mV. You can also fine-tune the input DC offset, also digitally from the FPGA (PWM baby!). Now it's just fine-tuning. There's a bit more noise than I'd like, so I'm trying out a 4-layer board with a dedicated ground plane and some more intelligent routing of the ground current returns.
The software can continued to develop. In addition to letting you adjust the gains and DC offsets per channel, there's also a new FFT window! It's all just python, so easy to expand the functionality.
We hope to launch this project VERY soon. Thanks for your support!