It's clear Starmer needs to establish Labour's position on Brexit, it's an easy line of attack for Johnson. This is what I think their position should be.

Firstly, don't campaign to rejoin. There will be time later. Now he would just be attacked for "denying democracy"

Outline Labour's plan for a deal. Close alignment, part of single market to restore free movement, but make use of Article 7 to limit inward immigration to those who have a job & medical insurance. Point out the Tories failed to use this in 10 years.

Can join CU too but if not a comprehensive tariff free FTA, which still allows us to negotiate our own FTAs, puncturing another of Johnson's balloons.

Now, the attack. Johnson will try to claim Starmer is trying to deny Brexit people voted for, thwarting democracy.

No, he's delivering Brexit. Johnson has failed to get a trade deal. What he is actually doing is destroying Brexit. He's letting down Brexiters by ensuring Brexit will be a disaster.

Starmer is getting a strong deal that the EU will accept, and thereby making a success of Brexit

This deal would ensure the GFA would be preserved & protected, be welcomed by the DUP, Wales & Scotland, and thereby strengthen the union. It would also increase chances of a FTA with the US.

Gives the benefits Johnson promised while minimising the damage of the mess they made.

This should be the position of @Keir_Starmer. It sets a path to rejoining at a later date but for now it'll satisfy enough Remainers & soft Leavers to cause a huge problem for Johnson. And focus on the message:

Johnson is making Brexit a disaster.

Labour will make it a success.