My bet is that the alternator or regulator failed, or partially failed. Maybe just as you took off ie after your initial checks. Flying around with a load of avionics etc on drained the battery.
I had exactly the same thing happen. I now check the battery voltage after start and make sure it goes up when I switch the alternator on. In my case it was the regulator that failed.
Recharge the battery
Measure the battery voltage when recharged.
Run the engine and switch everything on and see what the voltage has dropped to.
Switch on the alternator and check that the battery voltage goes back up a bit.
You may need to rev the engine for this. If it doesn't go up then the alternator/regulator isn't working.