I want to keep my X on iOS 11.3.1 so I can keep my jailbreak but want to get all my current convos/app arrangement/bookmarks and text replacemt keyboard shortcuts. None of this stuff synced from iCloud.
The backup/restore process does not include the operating system itself. You can restore a backup to a different device provided the new device is running at least the same version or newer of iOS as the old device. For example:
iOS 11.0 > iOS 11.2 = Yes
iOS 11.0 > iOS 12.0 = Yes
iOS 12.0 > iOS 11.0 = No
You can manually re-install another version of iOS as @rain111 mentioned, but only if Apple is still signing that version. They typically stop signing older releases a few weeks after the latest ones are released.