The tricky part is coming back onto the UI thread when the background work finishes. Your transcription thread has to somehow trigger the UI work to be done on the UI thread.
It seems the way to do it in Qt is with signals and slots, emitting a signal from your QThread and binding it to a slot in the UI thread, making sure to specify a "queued connection" [1]. There's also a lower-level postEvent method [2] but people disagree [3] on whether that's OK to call from a regular Python thread or has to be called from a QThread.
So I would try doing it with Qt's thread classes, not with concurrent.futures.
