This might not work because of the ITV Breakfast contract, but I think it might be worth dropping Lorraine on weekdays (have Lorraine on weekends, or Fri-Sun), and extend GMB - it would allow time for the show to breathe, including a proper sport and entertainment slots, a decent length celebrity interview(s). These issues aren't only a problem when Piers is on, it's everyday.
I've long wondered whether moving Lorraine to an afternoon slot, and maybe giving her a studio audience, would work. Like you said, it would allow GMB to breathe a bit more without being rushed off the air mid-interview, as is often the case. And, not that Lorraine necessarily needs it, but it would freshen up her show and inject a bit more life into it, after 25 years or so of the same thing.
Obviously with Lorraine sharing studios with LW it would throw up some logistical issues.