Two party systems are the problem here, not the candidates

Notably Corbyn actually got more votes than Starmer in both elections he ran - the change is more to do with the right vote being split between the conservatives and reform, the conservatives losing the centrist vote after the clown show of the last 5 years, and a unofficial pact between Labour/LibDems prevented splitting the centrist/left vote the same way the right vote was split.

the fact that they were able to make that pact is probably because they're not miles apart and Corbyn couldn't get that to happen. e.g. I don't think I could ever have voted for him given his attitude on Ukraine but I could have voted for Starmer if that was what made sense in my area.

Sure, but people like you weren't sufficient to change the fact that Corbyn still got more votes.

The libdems of 2024 have a very different situation in that despite the Tory implosion they are weaker than ever. I'd put my money on them either still making the deal or getting buried if Labour had even more votes.

I think you could posit the opposite - that he lost because he motivated more people to vote against him.

Labour now hasn't united its opposition because I think nobody fears it much.

That's just not true, though. Starmer has only increased Labour's vote share by 1.7%. He didn't significantly outweigh the loss in votes by demotivating opponent vote. Tory voters were more likely to vote for Reform or the Lib Dems than stay home.

Labour's opposition not being united is probably more attribuable to the Conservative party itself not being united. The amount of voters leaving the Tories to go to the Lib Dems and Reform has steadily been increasing.

Besides, a significant part of motivating people to vote against Corbyn came from Starmer's wing of the party itself.

Labour's share of the vote hardly changed from 2024 to 2019; fewer people voted Labour in absolute numbers.

Labour's electoral prospects improved because of the implosion of the Tories, not anything they did.