I do not know if ArcGIS Online supports large feature attachments; I've asked that team to verify. It may only be possible through the REST api or specialized apps. The Story Map crowdsource app should have all the standard limits in place.
There are a variety of technical reasons you cannot upload large attachments in a single request as you are attempting. Large requests have a high probability of being interrupted and failing. It also takes significantly longer to upload large attachments as a single request rather than multiple parallel requests. For this reason, you should look at the documentation links I provided above concerning multipart uploads.
Because I'm not as familiar with multipart uploads on feature service, I would recommend asking followup questions to the feature service team. At this point you are working directly with the REST API and possibly third party file upload libraries which are outside of the scope of this story map.