I don't think there will be a horrendous second peak or wave or whatever.
I'm not sure about horrendous, but looking back at other pandemics a 2nd or 3rd wave, using Drivingdown's term, seems possible. Looking at the graphs for many places' 1918-19 experiences wave 2 was the strong one and wave 3 often greater than wave 1. Not that this will necessarily happen with the current pandemic, but who knows?
Whether there are subsequent waves is to my mind down to the virus - it's nature and how it evolves - far more than how we respond by street-partying through to entombing ourselves. We can do things to modify the impact, but if it IS going to slosh around the bath, or turn endemic, that's just how things will be.
In the UK we have acted as if something that disproportionately hits the old old, those with diabetes, COPD, hypertension and so on is a serious risk to everyone. It isn't. Those off the risk list might have a bad roll of the dice, but it's a long shot. [Not that it can't be an unpleasant and scary thing to have, it certainly wasn't a fun thing to experience when we had it.]
I'd like us in the meantime to sensibly get on with life, not cause health, economic and social damage by overreacting and responding in ways that don't target those at risk and see how it all plays out.