Obviously the ones in question have met the requirement for D Towers. I see absolutely no need for anything other than G over the top of them.
Exactly. The problems at the specific locations are at and in the vicinity of the airodromes, hence the need for Class D zones. I'm yet to see any justification to apply more restrictive airspace above them.
The Aussie system of using a Class D Controller who sometimes is very busy to also be responsible for vast amounts of Class C airspace is clearly an accident waiting to happen.
The people qualified to comment re ATC workload and whether such airspace management is unsafe are ATCs, not pilots. Over the many years of such operation, I'm yet to hear any ATCs comment that the current practices at places like LT HB AY etc. are unworkable and unsafe.