How can I inject a taxonomy menu as the sub-menu of a standard menu item? I have a main menu, with a top-level structure similar to the following:

Home - Categories - Something Else - ... - Contact


Each of those top-level links goes to an appropriate node page.

I have a vocabulary for Categories (multi-level), and I need to get those taxonomy terms in a sub-menu under the "Categories" item. I'd like to avoid hard-coding the items into the main menu, as the sub-menu should grow/shrink as the vocabulary does.

In the past I've written over-elaborate code to accomplish this, which hooks into taxonomy operations and manages a particular menu's sub-items based on that (very similar to what Taxonomy Menu does, but without having to have a different menu for each vocabulary). 

While that method works, it's not ideal and certainly not generic. I can't just lift that code from the old project into the new one, it would require a reasonable amount of customisation. 

I can't help feeling there must be a 'standard', module-based solution to this.

I've seen some suggestions out there to use a bunch of different modules in combination (even some suggesting to use Views-based modules, weirdly enough). I'm open to any suggestion, but if it involves installing more than 1 or 2 modules, and a load of hard-coded, context-specific configuration, it's probably not going to help too much in this case.
Option a) Unless I am mistaken, can you not just pick where your Taxonomy Menu will be parented (Menu Location) when you create it? The only problem I think you might get with this is I think it requires you to rebuild the menu every time the taxonomy is updated

Option b) I have done something reasonably similar to this on my site, I used Menu Views module, to allow me to put a view into my primary menu.

If you create a new view which lists all of your taxonomy terms, as a block display, apply any additional filtering/modification in the view if necessary (you may not but in my example I wanted to). Then when you add a new link to your main menu, you can select to use a view rather than a link. From there you pick which view you want to use, select the 'Category' menu item as its parent and save (after saving you then have to enable it in the menu, which appears to be a bug with the module itself)

I can't guarantee that this is the best solution for you, but it worked when I just tested it