Confirmed that iNaturalist is totally fine with uploading older observations.
https://forum.inaturalist.org/t/can-i-upload-older-photos-as-observations/3259/3
Tried out the Flickr importer. It’s a bit rough for selecting, but it imports the description, time & location along with the photo, and you can import batches at a time.
I don’t want to totally flood my profile, but I’ve got a lot of timestamped photos of animals & plants since I bought my first digital camera…