I killed a PD using about 100k worth of steel and AFK ones only require frigate spam for 5% that should be scripted. Once PD is damaged you can do whatever you want. I think killing active PD requires about 5x the steel on the planet and any reinforcements. Killing active overlapping PD is impossible. Scout defence was important but theres a difference between sending 10 planets worth of fleets to kill an Oldry pimple and dealing with great wall of china PD.
Like you pointed out it is possible to defeat PD using much less force then I tend to use, co-ordinated attacks against grouped PD are more expensive, but still quite possible. Of course you need to time the attack in this cases, as significant reinforcements can really create a problem. Sometimes you also need to "pick" off the planets on the outer rim of the PD defense one at a time. Of course this costs you resources, but sometimes clearing the sensor path to the central planet means this pays off.
This is what I had to do in my last war. Five PDs had area's of overlap, with good sensor coverage from outlying planets. I had to take a number of outlying planets first which both gave me a clearer path at the PD defended planets and also reduced the defense fleets, and then sent 3 fleets of 100 battleships at each of the PD planets. This initial wave took out 3 planets. After that the remaining two were much simpler.
