I doubt if Corbyn is happy about any of Starmer's policies.
It's been suggested for a long time that he might try to lead a new, socialist party. If he did, I doubt if it would be anything like as successful as his supporters think it would be. No 'new' party in my lifetime has ever been successful, it might gain a few seats in a couple of parliaments but that's all.
What it would do, of course, is split the left of centre vote. This might let the tories back in, though they have their own problems to contend with in the shape of the Reform party, or it could lead to a greatly reduced Labour majority, or even a hung parliament. Which would be interesting...