Vee-tail
It seems to me that your problem is not really about gliders, it's about having many other people flying on the same day as you. Imagine replacing each one of the gliders on a good gliding day with a PA28. Would it be any easier or safer for you? Don't think so. Would a basic service ensure you didn't hit any of them? Absolutely not. Could ATC provide you with a traffic service? Doubt it. So your statement "Equally as a GA pilot I might wish to be free of the threat posed by gliders operating in the area of uncontrolled airspace I need to transit through" becomes "I might wish to be free of the threat posed by other aircraft operating in the area of uncontrolled airspace I need to transit through".
The things you do to ensure a safe flight - "each flight is planned to take account of weather, NOTAMs, terrain, danger areas, aircraft serviceability, etc" - Of course, and I do all that when I fly my glider. What's different?
And glider pilots wear parachutes because they routinely and deliberately fly in the close vicinity of other gliders, which as I pointed out earlier, is where the risk to them is. The only powered GA / glider collision I can think of off hand in the last few years is one in which the cause was found to be inadequate lookout prior to aerobatics by the powered pilot - it's not where the collision risk is.
So if your personal risk mitigation strategy is not to fly when it's busy, then fine, that's your call. But recognise that it's because it's busy, rather than that the other aircraft are gliders. And if you really want to avoid the gliders, you can always fly in the places where they are less likely to be.
Paul