There is, apparently, a seat in Manchester area with a Labour MP who may resign because of ill health. I doubt if he'd lose that one.
He was an MP for 16 years, has had government posts and seems to be making a success of his mayoral post. I'm not sure why anyone should think him less competent than Starmer who only became an MP in 2015 and who had no experience in government. I would also think that running Greater Manchester makes him more likely to be closer to the general populace than a Director of Public Prosecution.
I certainly like the principles set out on the Mainstream Labour website; they're far more what I would expect from a soft left Labour politician. Do people think he would renege on them once he got the job in the same way that Starmer did?