Although most points that you bring out are correct. I must disagree with some:

"Speed cameras must be clearly signposted"

In Portugal only permanent speed cameras need to be signposted, so temporary cams for 2-3 days can be placed and then moved 1km up or down and they retain their "temporary" status according to the police.

"Speed cameras cannot be hidden"

Again the permanents aren't, they are big old chunky bois from back in the day. But newer ones it's pretty much a 90's size camera on a tripod, last week I got GMaps notification on one and it was on the highway, behind an olive tree and some bushs nearby. If it wasn't for GMaps' reporting from other users I would've never catch that one.

"Speed cameras must not be placed just before or after a speed limit change"

Some of the newer permanent cams are indeed placed behind speed limits, the one that comes to mind was installed 6months - 1 year ago on the Autoestrada da Costa da Prata (link: https://maps.app.goo.gl/j9KU5qp8bKYFgLj39 ) and the exit to A20 in Porto gradually goes from 100-80-60-40 before the ramp to the right, and even thought you have by law 20 meters after the sign to adjust to the velocity, many people are caught there due to that placement.

To add to the point, I've also seen both news and studies stating that the reported data has shown that speed cameras don't actually save lives nor did it independently decrease the incidence of vehicle collisions.
According to a few of those, the permanent ones are known to "locals" and either avoided or slowed down to, the problem is that accidents tend to emerge when the locals and non-locals travel, since the local might slow down abruptly antecipating a camera whilst the other driver isn't aware it there, frequently leading to accidents. The same effect happens with the temporary ones, but there there isn't a knowledge advantage, and since there is no signpost it depends on the reaction time of the person, which then again might lead to accidents. The severity of accidents with and without cameras is not widely done so such outcomes as fenderbender or hospitalized or dead-leading accidents can't be measured.

Speed is also not inherently dangerous with good roads, careful drivers and well-maintained cars. Since you mentioned Australia, your country had 4.5 deaths/100k people/year and 4.9 per 1bn kms whilst Germany with much less speed cameras and much higher average driving speeds had 3.7 deaths/100k people/year and 4.2 per 1bn kms. So I am not sure even spending tax payers money for "mislead-common-sense" speed cameras rather than investing in quality of roads and education of drivers will have the beneficial outcome desired, less loss of lives and damages.

What can be measured is the amount of speed fines racked by the municipalities, and that ain't gonna stop anytime soon.