One feature I love about the FTDX-3000 is that there is a "tune" port where you can attach a switch like a foot pedal for tuning an antenna tuner. Can the Flex 6400 do this? Was really handy for tuning your amplifier / antenna tuners.
I'm sure you know that there's a TUNE button on the SmartSDR panel with a presettable power level. That is convenient and there is also a TUNE button on the Maestro. But I'm not aware of any hardware method of doing that like with a footswitch.
I also have a FTDX-3000. Nice radio and I blow the dust off of it every so often.