It does not need to be sysboards. The "SysBoard Controller" also handles joysticks, several of the Phidgets boards, and some Pololu boards. As well as an interface to FSUIPC. It also has direct plug n play support for the FDS CDUs, both USB and ethernet versions. There is even a "plugin" API if you are so inclined to program your own hardware interface. I wrote a plugin for Sim-A in Visual Basic to talk to my ancient EPIC hardware system!
Your parking brake switch and fuel lever issues would be a very simple fix in the controller as you can assign both the open AND closed switch states individually.
You can also run an instance of the SysBoard Controller on client computers for any interface boards or joysticks connected to other computers.
One huge advantage of using the SysBoard controller is you then get the Sim-Avionics system logic, which is what you are paying for 😉
Sim-Avionics communicates to FSUIPC. Latest Sim-Avionics version is 2.102 (as of last week).
Just a suggestion of course. I'm just trying to help. Sim-Avionics is very powerful.
Good luck on your build!
James (in Northern California)