We use the Steinberg UR series for testing, I think the UR12 is the least expensive one. It supports many sample rates and Music Studio on an iPad Pro will switch to 44.1kHz when it’s connected.
However, I cannot say for sure that this would resolve the crackling issue that you are experiencing. The crackling may also come from the fact that the iPad’s CPU reaches its limit when too many IAA apps are running simultaneously.
A possible workaround might be to increase the latency (buffer size). Music Studio doesn’t have a buffer size setting, but it uses the buffer size of audio apps that are already running. In fact, iOS lets the first audio app that you launch decide the buffer size, and forces it upon all other audio apps that are launched thereafter. So, what you could try is to check if any of the IAA apps you’re using has such a setting, launch it first, increase the buffer size / latency, then launch Music Studio.