Photo library app recommendation
Photos

I know the point of the question is an alternative, but I've been through both transitions: first iPhoto to Aperture, then Aperture to Photos. I think there's a few points still to be made for using Photos.


  always-on sidebar


I've always used the always-on sidebar instead of the toolbar since the very first version of Photos. Yes, the sidebar option was buried in the menus, but it was there and is much better than the toolbar. All that's happened in the most recent version is the removal of the (previously default) toolbar navigation and the activation of the existing sidebar option.


  moments, still no user-defined events structure


This definitely was one of the things I worried about moving from Aperture to Photos. However, Albums works almost identically to Events, and I set up a smart folder looking for all photos not in an album as ‘Unorganised’ which lets me organise new photos into albums.


  lack of batch editing (still true except for rotating)



Make adjustments to a photo.
Copy the photo adjustments using ⇧⌘C.
Select as many photos as you wish to batch apply the adjustments and paste using ⇧⌘V.



  cumbersome GPS tagging


Feel free to expand on this one, but this is how I go for photos without GPS:


Select photos that need geolocation.
Get Info (⌘I).
Enter the location in the ‘Assign a Location’ field. The field accepts regular text search (uses Apple Maps) or latlong coords.



  monolithic file structure (i.e. no referenced files in user-defined folder structure)


Storing photos in the library definitely isn't required. Photos has full support for references files. When importing files, deselect ‘Copy items to the Photos library’ (this preference is remembered for next time). For more information about using referenced media, see Change where Photos stores your files.