Not sure if you will be able to use Strom and sync a synth app at the same time. Remember the RYTM can’t sequence other gear it can send MIDI start/stop info though. So if you wanted to use an app with a sequencer and sync the two, you would need something like an Oplab or iconnectmidi. This is to send the stop/start messages between RYTM and iPad.
I personally just sample my iPad synths and load them into the RYTM. iPad is on full time STROM duties from now on I think.