Johnson's desperate attempt to get Starmer to talk about Brexit at PMQs today was a complete vindication of the Labour leader's strategy and @J_Bloodworth assessment of it today https://www.politics.co.uk/comment-analysis/2020/09/09/competent-leave-starmer-is-side-stepping-the-tory-trap-on-br
Those calling for Starmer to get stuck into it should hang back, imo. Soon enough, deal or no-deal, he'll have to. But there is nothing to gain by getting heavily involved now.
It would be different if there was something he could objectively change at this stage. But there isn't. And in fact you could make the case that his intervention would make the government even less likely to try for a deal.
The real question is this: If the govt gets a deal, what does Labour do? That's going to be very difficult. Support, and you lose the ability to criticise in January when things start going wrong (they will even under a deal). Oppose and you fall into the culture war box.
Opposing also leaves you vulnerable to argument that you are effectively supporting no-deal, because of the timetable.
The truth is this, I think: The quicker the Brexit debate dies, the easier it is to make the case for EU membership as a change to the status quo, ideally/hopefully in advance of the 2030 election.