I'm not a fan of first past the post, but I can see why no Labour leader has ever really gone for it.Bizarre to me that Starmer doesn't see electoral reform as a priority. Especiay given that Scotland won't be delivering Labour a meaningful number of seats ever again. Then again, he's timid about pretty much everything. I think Burnham has said he would seek reform though.
Starmer might not get a choice in the matter if he's reliant on Lib Dems to form a government (which he surely would be). I expect the Lib Dems wouldn't make the same mistake again of allowing a watered down version of PR to go to the public. They will likely dig their heels in this time.
But anyway, yes I'll be voting Labour whilst rolling my eyes and sighing. This version of labour doesn't represent my own politics and i don't trust Starmer to deliver the meaningful change we need, but the Tories just have to be stopped right now.
Under PR Labour would usually get less MPs - it's no shock that the Greens and Lib Dems are desperate for it. It's also a bit of a myth that there's always been a big anti-Tory majority - even with PR we'd likely have seen 12 years of Tory rule. I am sure we'd all have loved the Tory/UKIP coalition of 2015 :/
Your post also suggests that you're to the left of where you think Labour is now. While full PR might result in some success for a more Socialist party, it's more likely that any Labour party in power would be dragged towards the centre by the Lib Dems (who remember went into power with Tories in 2010, and explicitly ruled out any kind of coalition with Labour in every election since), and with a much bigger PR powered Lib Dem party any truly radical policies are likely to be watered down.
Whatever we may hope for, PR is more likely to take us to the centre, and given how popular Brexit and Boris's populist bullshit campaigns were, there's arguably more risk that we'd end up with a populist right wing government under PR than any progressive utopia.