iCloud

The apps won't be removed/affected. There may be some data loss, but you can minimize it. If syncing photos with iCloud, ensure Optimization is turned off in Settings/Your Name/iCloud/Photos. Then make sure full resolution photos are on the device/computer. When signing out of iCloud, if asked whether you want to retain data, choose yes.

If you aren't syncing Messages with iCloud, you should retain the messages.

For photos, read my post above.

In the screen shot, slide all four items to the right.

If you aren't syncing Messages with iCloud, you should retain the messages.

For photos, read my post above.

In the screen shot, slide all four items to the right.