Excluding genres from shuffle

I suppose I could make a playlist of my entire library excluding the genres that I don't want and then apply shuffle to that playlist

That's exactly what I did. I've made a playlist that only contains music and does exclude a few genres. Here are my settings:

Match all of the following rules:
Podcast - is false
Kind - is not - mpeg audio stream
Kind - does not contain - movie
Genre - does not contain - audiobook
Genre - does not contain - comedy
Genre ......... exclude a few more
Live updating is checked

but that seems an awfully cumbersome/time consuming way of doing it...and it would need modification every time I add to my library

Once the playlist is created (taking only a few minutes), you will hardly open it to make modifications. At least, that's my experience.
If 'Live updating' is checked, the rules will automatically be applied to new additions to your library.

Hope this helps.

M

Or you could select the files(s) you want to skip, ctrl-click them, then select "Get Info" (or just command + i). Then go into "Options" and select "Skip when shuffling".