I'm guessing a Hi-Z input before this contraption could improve the tone and do the pickups of the guitar or bass justice. A dedicated buffer pedal or just a pedal with buffer bypass perhaps?
Yes I was surprised to see the guitar signal going in directly. A simple DI box would improve the sound noticeably.
Most definitely. A radial reamp DI would improve the tone on this rig noticeably. Even if you didn’t use reamp specific hardware, using 2 direct boxes to match impedance on the input and output of the Pi would help.