What would collective acts of grief look like? Entire processions marching in black? Vigils for those that died of covid? Twitter bots listing their bios and faces for each victim.
We can write entire books on the victims of 9/11 but we have the equivalent of 1 9/11 in victims every month or so and we shrug our shoulders.
I think you need to look at the mourning for the queen.
Vigils, minute silences, any large events that can be convinced to hold a silence or a moment of rememberance, etc etc. You could hold events specifically for the purpose of rememberance, target people that have been effected, target old people's homes, nursing and healthcare, millions and millions of people were directly affected and all of them could get something positive out of a moment that directly connects them with the millions of other people that were affected.
There is definitely collective solidarity building that can be done through this. I don't see it as idealism as some others are arguing here. People's emotions and connection to the wider collective is a real material thing, the isolation everyone feels is not idealist it is real factual isolation and anything that connects people in solidarity and a collective way pushes back against that isolation.