Having used the interface for the first time today, I hope it's okay if I give some feedback about the tool and its documentation.

Firstly, I'm extremely glad this exists and that it is set out so clearly. I find it incredibly helpful and my only suggestions are relatively minor.

1) Accession number:
My metadata has a couple of identifier fields: a shelfmark which identifies the object in the GLAM's catalogue, and a file identifier which identifies the digital image in the GLAM's database. The shelfmark is, from the GLAM's perspective, the most important single piece of information. It seemed sensible to use the Artwork template and put these identifiers into the Accession number field. However, when I tried, the field just didn't display (in the GWToolset's preview of the first three records). Maybe there was a problem with combining two source fields into that one field? Or maybe the Accession number field is expecting a certain format?

When I instead assigned the identifiers to the Notes field, they displayed fine, and having them visible is the important thing, so it's not a huge problem.

2) Global categories:
I chose a category which includes an apostrophe character:
Files uploaded by the Bodleian Libraries' Wikimedian In Residence
The system renders this as:
Files uploaded by the Bodleian Libraries&
This is easy to fix post-upload with cat-a-lot or a similar tool, but it would be nice to fix it in code.

3) Entering a template in a field:
There are different ways a template can be specified: by its name (e.g. cc-by-sa-4.0 ), by wiki markup ( {{cc-by-sa-4.0}} ), by its name with namespace ( Template:Cc-by-sa-4.0 ) or by a web URL ( https://commons.wikimedia.org/wiki/Template:Cc-by-sa-4.0 ). When I put a template in the form, it's not always clear which of these formats to use, and in fact the answer is different for different fields. With a bit of experimentation it becomes clear, so it's not a severe problem, but it would be easier if the form was more clear about which format to enter.
For the License template field, the form gives an example of {{Template:Cc-by-sa-3.0}}, but the Template: namespace is unnecessary.

4) Partner templates:
For the upload I'm doing now, I'd like to have *two* source templates. It would be welcome if this were a text box taking multiple lines of markup, rather than a text field. Again, this is no great problem to correct post-upload. Maybe more than one source template can be put into that box? If so, the form should say so.

5) Item specific categories:
This is merely something that would be *nice* to have in the long run: having the phrase *after* the value of the metadata field. My metadata has lots of keywords, such as "Crowds", "Fish" or "Singing". It would be nice to auto-categorise these into Category:Crowds in art, Fish in art and so on. Commons has many hundreds of such categories.

So those are my suggestions. To emphasise, my impression of the upload tool is very positive.