Faster autofocus - what's available?

As Deep Thought would say... "hmm, tricky". I'm glad the AF assist is helping. I've been known to resort to live view when the AF is really struggling, but it's tricky with a moving subject. I often try to find a nearby high-contrast edge (usually edge of face against the background) and rely on depth of field to cover the eyes - but for me, the problem is often shaded faces with a bright background, so there's something near the plane of focus that I can find. For what it's worth, I find a lot of people are still moving at 1/60, so I tend to find myself pushing the shutter speed (and therefore ISO) a little more; third-party software can do quite a good job of denoising, but motion blur is a pain. It may not matter so much either way at smaller sizes, though.

If the subject is wet and you don't want to use the speedlight AF grid, I'd suggest a small and fairly bright LED light (I have a Thrunite Ti3 that runs of a AAA; I can also recommend the Fenix LD02 - other small and bright flashlights are available, but there's a big difference between the premium models and the no-name versions). It's got different brightness levels, the brightest of which is pretty decent (especially for its size), and you can put a fairly subtle highlight on a wet subject, which should give the AF system something to lock onto. Just thinking it might be less obtrusive than a bigger or brighter light source - and you might get some catchlights in the eyes. They're only a few dollars if you want to try it.

For what it's worth, I think I'd be expecting any of the MultiCAM 3500 cameras to behave a bit better than you're suggesting, especially with an f/2.8 lens, but it's hard to say without being there. I'm sure others here have more experience in challenging shooting scenarios than I do, so I'd take their advice over mine. :-)