Here's an optimistic scenario how the campaign might turn out:
-Brexit is on 'pause' and there'll be no new news to keep it in the headlines. There's already a lot of Brexit fatigue out there, so maybe voters might not be as pumped about it coming into the election. It's the end of the year, its dark out, and we're all exhausted. The Dominic Cummings approach has been to turbocharge Brexit, create maximum heat and ride that boost. But maybe that peaked too early and there's not enough Brexit outrage left to keep the momentum up - especially as it was Johnson himself who put the brakes on.
-If that's true then there's more room for the domestic agenda to take a central role. The Tory election pitch is to throw a lot of money at hospitals and schools (whether true or not is not the issue). But hospital and school workers basically hate Johnson, meaning he can barely step foot in one without getting booed or similiar. In fact public appearances are going to be awkward full stop. By contrast Corbyn will get a much better reception. Over time this could become a clear pattern in media representation. Heckling Boris might even become a Thing.
-Corbyn has already had every possible arrow fired at him - there can't be anything new to try and pin on him. Johnson by contrast has fresher rope infront of him. Might yet get hung a bit.