It's not possible to use an app as a replacement. I'm not sure you understand what ADP does or how it works. Whether you're using ADP or not, your data stored in the cloud is always encrypted at rest on Apple's servers. The only difference is who holds the keys to decrypt your data when accessed. With ADP on, the key is stored on your devices; Apple themselves cannot access your data. With ADP off, Apple stores the key. They could access your data if they really wanted to; or if there was a data breach, then your key may be used by others to access your data.
For a "real world" example, say you have a storage locker that's always locked. With protection on, you have the key in your pocket and can let yourself in; the security guard can't get in or let anyone else in because they don't have the key. With protection off, the security guard has the key instead. You verify with the security guard to let you in, but they could also let themselves and others in.
As Apple_Robert linked to, the only thing you can do is to stop using iCloud and keep everything local to your devices. (Or in the example, stop using the locker and move everything into your own house/storage space.)