Add keywords/location/description to a file w/o Photos app?

If you want to manage your photos in the Finder, you could add Tags to categorize them: macOS Sierra: Use tags to organize files

However, that would not "embed" the tags in the File. The only way to do that would be to edit the EXIF data, but I'm not sure there are fields for "keywords" in EXIF data. It also wouldn't be simple to search the EXIF data.

or, if I export modified images they gain some weight that's not related to having new info. File size isn't really a problem, it's the "it's not the exact same that I imported before" that drives me crazy.

Photos (and iPhoto) do not modify the original files. They create a copy and allow you to edit that copy so the orignal stays intact. You can "Export the Unmodified Original" if you wish.

The Editing pane has a Revert to Original button.

If you wish to expunge the original image after modifications, you would have to duplicate a modified image, then delete the "original."

With Aperture you could write the descriptions and keywords directly into the original image files.

Photos for Mac does not support this.

I am currently using Jetphoto Studio to write GPS, titles, and keywords into the original image files, before I import them to Photos for Mac.

If you like the Terminal, you could use ExifTool to set the EXIF tags of your photos. It is very versatile, but can only be used from the Terminal. I have not yet tried the available GUIs.

See: http://www.sno.phy.queensu.ca/~phil/exiftool/.

Photos for Mac does not write into the original files, but you can save the metadata as a sidecar file. If you export your originals with option "Export IPTC as XMP" enabled, you will find an XMP file for each photo with the GPS, keywords, titles, descriptions. Many image editors can restore the metadata tags from these sidecar files, when the import an image file and the sidecar is in the same folder-

GraphicConverter 10 (it is a functional trial) allows you to browse a folder of images in thumbnail view. With an image selected in Browse mode, there will be a Keywords menu item under the Window menu. When you select this, a small panel appears with some sample keywords, with the ability to add, remove, select, and sort keywords. When you do this, they are automatically written into the EXIF data as a Keywords category. You do not need to save the image, and can quit from GraphicConverter.

A Finder Get Info window will show these keywords for the image. In the Terminal, the mdls and mdfind commands can search for these keywords: (mdfind -onlyin ~ 'kMDItemKeywords == "*Landscape*"cdw' *.jpg) will find all jpegs containing the case-insensitive (the c part of cdw) keyword “Landscape.” The creator field of the image will be changed to GraphicConverter — though that may not be desirable should it replace the original creator (e.g. Photoshop, etc.).

Why not try this on a small folder of duplicate images as a test?

When you add a URL to the title or description in Photos and export the photo with "File > Export", the URL will show in the Info panel for the exported file in "More Info". You can copy and paste it, but it is not clickable.