I would reckon that four fatalities involving professional pilots like yourself would be a known safety issue.
Any reasonable person could write the report now.
“A do it yourself collision avoidance system when in IMC close to an airport with no promulgated separation standards is not safe”
And remember 6 dead not far away at Benalla with two professional pilots in the same type of airspace. In that incident the ATC did not warn the pilots that the route monitoring alarm had triggered a number of times because the aircraft was heading towards class G.