RPI will die in a couple of months, rad-hard electronics are needed,etc.. Also space-class amplifiers/transmitters are very expansive.
If this goes on, then I strongly suggest to pay some enterprise to launch it. A rocket per si is dangerous enough, a home made rocket is very risky. Also you will have a lot of troubles with government and airspace clearance to launch.
a RPI would survive longer in a protective environment (I.e inside the satelite) i also think that a RPI or, as you said a ''tweaked'' one would be a good idea for this, considering you could save like 5k Euro on it haha.
as for the structure you posted, it's a good start.
i will be fun to see where this goes, would be fun to see BTC in space