Don't conflate SDR, phased arrays, and MIMO. They're separate things. The latter two can be implemented vi a variety of hardware. State of the art radars tend to use a lot of FPGA's for aperture synthesis, not SDR.
SDR also isn't quite the panacea you're presenting. You can't just slap some magic processor in something and then gain access to huge swaths of spectrum. Even highly advanced UWB phased array apertures are limited to around 10:1 bandwidth. Getting up to and past 1 gigasamples per second is difficult even using the prior mentioned pile of very expensive FPGAs.
You won't be able to pull in everything from ~100mhz signals to 18ghz signals all in one $300 pizza box on your roof. The spacex device will be much more narrowly targeted.