Are there any add-ons that can help speed up the process of adding large batches of images to an EE channel (that is, each image gets its own entry, rather than a gallery-per-entry situation) and applying specific tags to each batch of images? Or is the best approach to get Zenbu & Henshu to edit the entries after the fact? Does it handle tags?
I've been using Solspace Importer for something similar to what you want to do, and it works well--and accomplishes a big chunk of the batch processing that I need.
However, in my case, after I run Importer to create and populate the entries, I need to do some custom SQL to translate the image file names that come from my spreadsheet into actual file entries in Assets, and into corresponding references in the Assets fields in the EE channel entry data.
So, if you use an import add-on, part of what you have to account for is the type of field you're using to store the image references. Each field type has it's own data structure in EE, albeit many store simply text in the regular EE channel data, and so don't require any special handling.
The above add-ons handle a bunch of different field types between them, so one or other may handle 100% of what you need. Otherwise, you might need to do some custom processing.
And, otherwise Zenbu & Henshu are probably the best way to go.
I have a similar requirement on a website (one image per entry and large number of uploads), I've tried several different solutions, including moving the galleries to a separate cms(koken), and here's my summary for you.
Currently there isn't a out-of-the-box solution for this type of gallery, unless you can somehow use the spreadsheet solution described above. Your solution is to choose either Assets or Channel Images(or other bulk uploader that I haven't tried) and write custom code that takes the uploaded photos and puts each one in its own entry. Then using Zenbu and Henshu you can do bulk cataloguing. There's a new version of Henshu in the oven that is perfect for this.
While choosing between uploaders you have to consider that some use GD library and others Image Magick, GD tends to screw up ICC profiles and that's why I looked for and external solution like Koken(and also for bulk cataloguing features), since then Channel Images started supporting Image Magick and with the new Henshu capable of the editing I need I'm moving back to a full EE gallery.
I hope this helps.