I understand the political calculation of Starmer's Brexit position as a means to get power, but the global situation with the US gives Starmer the political cover he needs to just full on reverse brexit and have the UK rejoin the EU as a full member. Trump has given him a narrow window of opportunity to correct an obvious wrong. Starmer needs to seize that opportunity, NOW!
Sure the UK won't get the same cushy deal they had as an EU member before, but it'll still be better than anything he's currently proposing publicly.
Politically it might cost him any Leavers who voted Labour (if any are left who haven't defected to Reform by now), but he'll earn some favour with Remainers who left Labour for the LibDem, SNP and Greens which might be enough to secure a minority Labour government. And, a Labour government moving to rejoin the EU could be enough to shoot the SNPs hopes of a majority in Scotland this spring and their promised independence referendum if they get one. Not that I support any of that, (I've never voted Labour and disagree with most of what comes out of Starmer's mouth). This is just the political benefits I see for Labour if they choose a different path.
Anyway, TL;DR: DO IT! Rejoin the EU today!