If something is free, then you’re the product. Your data will be used for whatever the storage provider would like. For example, Google Photos was initially very generous with storage, because they were using your photos to train their recognition software. Gmail is reading your emails to build our your advertising profile. Your data is a commodity, and your privacy, if you protect it, is valuable.

The best way to do what you’re asking for, to ensure privacy and security, is to self-host. There’s an initial cost, and some maintenance fees over time, but you’ll also be investing in learning valuable skills in your free time.