Set up a Google Photos shared album, and an equivalent iPhone one (don't know how it works there, but presumably they have one?) Let guests choose which one to upload to. Do some hourly backend sync between the two. https://www.multcloud.com/tutorials/sync-google-photos-to-ic... Also, congrats!!
I hadn't considered setting up two shared albums to meet people on their platform, so thanks for that idea!
https://news.ycombinator.com/item?id=38431743
within the comments, there's people also complaining about losing photos as well. just so you're fully informed