The relay config only means beast splitter will be pushing any data it gets into dump1090-fa via 30104 while also offering them on 30005.
beast-splitter INPUT is configured like this:
lappend relayconfig "INPUT_OPTIONS=\"--net [$config get receiver-host]:[$config get receiver-port] $force\""
So it connects to the IP receiver-host on the port receiver-port. (Even if you would enter localhost:30104 here this would not work as dump1090-fa only accepts but does not provide data on that port)
dump1090 is configured for net-only and bo-port of 0:
lappend receiverOpts "--net-only" "--net-bo-port 0" "--fix"
Data makes it way to dump1090-fa but not to beast-splitter or piaware.