I have to admit that this 'Theme Project plus Theme Keyword' thing is a bit unusual. It's happening because you're doing projects based on themes rather than shoots. If there was an easier way to structure on shoot would you do it? It won't work unless your originals have some basic metadata already. Might not work on scans for EG.
click on 'Photos' right under 'Library'
put viewer into list view
click on date column
you're photos *should* now be listed in chronological order
select a range by the month or something
drag those into a new project named by that date range
Do this as much or as little (one project idea) you'd like to have the projects broken up, and then keyword by theme. A photo could have as many themes as you want. Make smart albums that show the themes.
Another way, which involves starting again from scratch. It would eliminate your duplicates and make most of this automatic instead of manual. But again, the originals will have to carry a date stamp already. Most likely they do.
'Import'
nav to a theme folder
'Do not import duplicates'
uncheck all
put import window in list view (middle of the 3 buttons bottom left)
select a date range
click one check box (all selected should become checked)
select theme keyword preset (might as well add copyright etc)
import into project named by date range
The only manual sorting/keywording that you'll need to do is to find the photos that have more than one theme. I'd likely do that by clicking 'Photos' under 'Library' and selecting multiple photos to keyword. I'd probably do this in Viewer mode so I could see as many thumbnails as possible at once. Or maybe I'd work in a theme album and see which of those have more than one theme.
Ultimately only you can decide what to do, but If I were you I believe I'd start over from the beginning.Then you'd have project strictly by shoot with themes by keyword and nothing duplicated. If in the future you decide a photo has a different theme, or a new theme it's very easily done.
I'm not trying to monkey-wrench your plans. I just think this will give you a library that's easily managed and makes the most sense. And that's all it is really. Your project by theme idea is fine if it works for you.
Just some more food for thought. And maybe a few pointers on how powerful the metadata tools are.
Charles