@Oneeightnine The only horrific thing he has to do is look like he'll lose the election.
For what it's worth, I too have no confidence that Sunak will win the election, thank goodness.
Those submitting letters to the committee now probably never liked Sunak in the first place, some may have even submitted them on day one when they were just angry at him ousting BoJo.
In the first term of a Labour government they will maybe be a bit less hostile towards the EU but they won't be re-joining or anything. They'll have no mandate to do so since they worry putting it in a manefesto would alienate their red-wall voters.
As it currently looks Labour won't do much of anything at all. Continue tory spending plans, refuse to raise any taxes, not rebuild any schools, not make any changes which may affect the social order or reform capitalism in any way.
Expect 4 years of calm with almost nothing happening. Which in itself will be quite a relief.