"From your experience, are there any workshops you'd particularly recommend"
We learned them in The Critics Group and used them extensively in London Singers Workshop (both long gone), but once we had mastered it, it was something we did at home.
Don't know where you are (or if she still holds them) but Frankie Armstrong was holding voice exercises at one time - I heard they were excellent
The voice exercises are 4 vowel-like sounds based in order to produce a 'pure, uncluttered sound'.
Then there were a number of sohort singing exercises in order to deal with accuracy in articulation, pitch, large, small, awkward tntervals, breath control.... pieces (a minute or so long) from Wagner, Gilbert and Sullivan, mouth music, Kurt Weill
You can probably devise your own exercises; I have recordings of the ones we did which I have been promising people for ages, but have never got round to editing into a usable form - am now doing so as I type - happy to let anybody interested have a copy when I've finished sorting them.
Can I say, there are plenty of others if you can lay your hands on them but they worked for us.
Also, one of the importent aspects of voice production is controlling tension - it's worth getting hold of simple relaxation exercises for this too.
Jim Carroll