Few things to add:
Regarding PFD's, anything helps even impact vests. Whilst there are many what if's of if you get knocked out in spots such as vicky point we are often not sailing alone there is a good chance you will float and can be revived within sufficient time. The current Hutchwilco PFD I have floats person upright anyway, as do many PFD2/3's nowdays. That raises the most important safety point- don't speed sail alone. Keep an eye on those around you, here at least we tend to keep an eye on each other anyway.
Hitting the aerotech xply/pentex as opposed to monofilm wouldn't make that much difference with both lying across the water. We're looking for flatwater anyway so it's going to be like concrete just with something dispersing the load over a wider area of concrete.
I've found the worst falls, like haggar's happen when you are travelling a little closer to the wind than you think but the board is running along steepish swell/chop. This lets the board break on the crest and catch down the wave face, or slightly less wild spin out and catch into rear of chop.
Learning how to fall is part of learning how to speedsail. Drop out of the harness asap when something goes wrong and either land on you're a$$ or fight it (which can leave you with sore ribs for a few weeks

). If you do get pitched forward try to rotate the sail so you not landing in it but going around it, if you fight it or pull back you are going to land in it.
I've had exactly the fall you've described Haggar. It was a once off, and I guess I was lucky that I collected the boom with my head instead of landing square on the sail. I was only doing 30knots at the time but it happened so fast I didn't even register before I found myself chewing the boom and feeling my neck creak. I do remember my next thought after it registered was expecting my neck to go snap.
It might take a while for you to get back on the horse after you are cleared to sail again, but i'd just put it down to freak accident and next time you might see the symptoms before it happens. You're misfortune is a timely chance for us to all consider safety and pushing beyond our limits before the worst happens.
For PFD's in Brisbane I reckon Goodtime has probably the best range. Surfconnect has stock/good availability of impact vests. Boardcrazy and Caloundra City Sailboards can get in the weight/impact vests, but there is a bit of a wait on these.