For what it's worth, Immich supports this too. You can create an album (for each event), create a shared link, allow public anonymous uploads for the shared link, and then give the link to everyone at the event, and ask them to upload their photos. It can be done from any web browser.
I would love to know if there's a way to secure this though. I'm not prepared to have people constantly trying to login to my immich instance so it's only accessible via VPN
You can use something like Immich Public Proxy to only expose the /share path of your server and keep the main /api path that has everything else behind VPN