When did you try hey calendar? I've never used anything from hey, but saw a lot of griping about its performance, and eventually saw that they apparently addressed the primary issues.
I was recently tasked with building a PWA to escape the play stores. A vitally important feature was notifications for the messaging system.
Hours of research trying to figure out how to deliver a notification through the PWA, even if the application was closed. Eventually I thought it wasn't going to be possible. But I remembered the HEY products were PWAs, and they had an email product. Surely that has the notification feature I'm trying to replicate? It's a paid service.
... no, I couldn't get their notifications to work, even though I enabled all of the proper settings. Crazy.
However, when I was about to give up and abandon the entire project, I had one of those moments, and got it to work.