Any SaaS or API for 1-on-1 file sharing?

Hi, We are making an app for our gym and part of the app is the feature for users to share their progress pics with their coaches. We do not want to build something on our own and we want to use an existing service for this to avoid the cost and also the risk and responsibility of handling user's pictures. Our plan A is to go with a chat service like Sendbird, using it for chat and files sharing between our coaches and the clients. But the only thing we use that for is text chat and image sharing and we will not use all their features that we are paying for. Plan B is to have the chat as text only but allowing the clients to upload their progress pictures on a space that is secure (end to end encrypted) and only the coaches that the client wants, gets the access to view them. Is there any Service/API/guide that we can use to overcome this. Thanks, Amin