I asked all of our friends to upload to a Google Photos shared album. Most of my friends use Android and most of my wife's friends use Apple-- didn't hear any complaints about Google Photos. Our experience was that most people have a google account anyways and are familiar enough with Google Photos.
The people I asked for advice said that they'd prefer to download Google Photos to upload/view photos vs. a third-party service that didn't require them to download anything (i.e. Synology photo request).
For Google Photos, make sure to set the "BACKUP QUALITY FOR PHOTOS & VIDEOS" option to "Original quality". It's the very top radio button under the settings (for my using Google Chrome on my laptop). Can't miss it.
One problem that I often encounter is that people either don't use Google Photos/iCloud, or they don't have sufficient free space in their cloud storage account to upload photos/videos. Additionally, in the case of Google Photos, if the person has configured Standard Quality for their personal account backup, the shared photos will not be in the original quality.
The people I asked for advice said that they'd prefer to download Google Photos to upload/view photos vs. a third-party service that didn't require them to download anything (i.e. Synology photo request).