Re-upgrade to macOS Mojave (esp. parts relating to iCloud) without loosing data, etc So my iCloud setup is brokenly, apparently because an incomplete macOS Mojave upgrade (from High Sierra). How can I "re-upgrade" to (reinstall) Mojave without loosing my existing data (e.g. in /Users/) and applications (e.g. /Applications/Audio\ Hijack.app/)?

For instance, is Command + R on startup (then "Reinstall macOS") the right entry point, or can I just repeat the part of the installation that is concerned with iCloud?
In response to the original question:
You can't just select parts of the system software to reinstall; you have to reinstall all of it but that shouldn't affect your mac.

To clarify about reinstalls/clean installs:
As far as I know/can remember there isn't an "option" to just reinstall/repair install mac os.
Apple actually make it simple - if you boot to recovery mode (hold command and r while restarting on mojave (especially with an apfs formatted drive- holding alt down to select the recovery drive doesn't display the recovery drive to select it anymore)). When the screen loads up and you select install mac os, if you select your drive with an existing install of mac os on then it will just reinstall the system files (including ONLY the apple software (terminal, textedit, iTunes, etc) BUT won't touch your data files, user accounts or the applications that you installed yourself and they should work as before).

IF you want to do a clean install (removing all of your own data and applications) you must first select disk utility, select your drive and erase it/format it. THEN it will install just the mac os and everything will be like factory default settings.