Starmer will struggle to implement worthwhile policy without a mandate because he'll be relentlessly attacked by the press for anything that shifts towards collective benefit.
As for Corbyn he was "genuinely" disliked because he was made the subject of a cynical, broad spectrum press onslaught.
Labour's 2019 platform was largely banal social investment: increase NHS funding, increase the minimum wage, stop raising the pension age, publicly funded allied health care, more effort to reach nett zero emissions, scrap universal credit, tax private schools, free or nearly free buses. Incremental, practical stuff.
The controversial elements were the re-nationalisation of energy and water, the post, telecoms and the railways, and a second Brexit referendum. None of which sounds too bad to me.