Folder-based photo management software for local use You'd think that there is more than enough photo management software out there. But for quite a while now, I have struggled to find Mac software suitable for my requirements. 

The management software will be used on normal everyday pictures (holidays, landscapes and so on) as well as a large collection of documentary pictures with little artistic value from renovating protected old buildings. Working with those documentary pictures will require extensive tagging and search functions.

I don't strictly require built-in editing and raw conversion functions.

My requirements list:


Native Mac program (or at least very well ported interface from a different OS)
Pictures must be stored in a folder structure on disk, not a monolithic catalogue
This folder structure must follow changes made within the software, and any changes made in Finder must be tracked and represented in the photo manager
The manager must be able to write geotags, textual tags and timestamp corrections directly into the images (in order to make the metadata available to 3rd party apps -- sidecar files like those used by Darktable are useless there)
Geotags must be viewable on a map
Metadata and changes to the image catalogs must be easily syncable using classic, non-cloud tools like rsync, unison, Bittorrent Sync or SyncThing. 


Bonus points for:


support of other operating system
hierarchical tags
user-definable integration of external editing tools


Apart from KDE's really great DigiKam, which suffers from a not-yet-perfect Mac port, I did not find any software fulfilling those rather simple requirements. Perhaps someone here can point out what I missed.

I evaluated the following programs:


Digikam. Great. It's just heavily using the KDE ecosystem, which makes building the port on a Mac hard. It's also badly integrated in the OSX desktop.
Aperture. That's what I'm currently using. Syncing catalogues between machines using rsync is not trivial. Managed libraries don't properly follow changes done in Finder. Tagging is unnecessarily complicated.
Lyn. Small, lean, Mac only. Rather rudimentary. Doesn't fulfil requirement 4.
Darktable. See no. 4 above -- sidecar approaches are impractical for me, and "darktable will never ever open source files for writing".
Capture One. Fails on point 3, and the support told me that syncing between machines should only be done through manual import/export of sessions.
Media Pro. Same as above.
Unbound. Very rudimentary. No external tools configurable. Needs Dropbox as sync backend.
Pixa. Seems to be developed only very slowly, which does not make me feel good about its longevity. Doesn't really seem like photo management, but rather more like digital asset management. No geotagging.
Apple Photos / Google Photos. Cloud based. Too rudimentary.


Edit: November 2019 additions


Skylum Luminar 4. Represents the folder-based structure on disk perfectly in its DAM component. Offers flexible plugin management. Can not handle geotags at all, however, and lacks a proper EXIF/ITPC editor. 


So far, those are my test results. Is there anything I'm missing / got wrong? Is there any software available that would fulfill my requirements?
You basically have written the summary requirements for Photo Mechanic.

It's a folder based, workflow tool that is cross platform and highly optimized for managing images. Whatever changes it makes are stored in metadata in the images so you can edit them elsewhere as you please and it won't break import/export like library based workflows cannot do.