Defense in depth is a fantasy in the game at the moment
Although I would agree that smaller groups of turrets aren't holding up as well as they probably should, I wouldn't go so far as to say defense in depth is dead.
Admittedly, I have not faced any AI >8, but I still practice some defense in depth. At all times after I get a strong enough economy, I keep at least 1/3 of my turret cap on my homeworld. Only if the closest AI planet is like a dozen hops away would I go below that, and even then it would never go below 1/4 of my cap. After all, if I lose my home, I lose!
On my core worlds (planets adjacent to my homeworlds), I place a little more than half the number of turrets I do on my homeworld, though I am more willing to reduce that proportion a bit to make room for other planets. (I make an exception for core worlds that are "landlocked" by the homeworld, where the AI would have to pass through the homeworld to make it to that core world. In this case, I treat that core world like a planet I don't care very much about.)
A little less than 1/3 of my turrets goes to my "whipping boy" for waves. A moderate amount of turrets goes to any "backdoor" planets to help take out any loose threat or whatever.
On the planet(s) right behind my whipping boy and right behind my backdoors I'll place a few turrets. On all other planets, I barely place anything, often times only one of each turret, or possibly no turrets except for 4 snipers.
On each planet, I place turrets not only around each "at risk" wormhole, but also the command station, giving a bit of defense in depth within a planet as well.
When properly supported with a fortress, tractor turrets, and gravity turrets, this arrangement can hold off most waves, especially if I use a military command station on the whipping boy. Only the waves that the RNG gave a good boost too would need some assistance from my mobile fleet.
This arrangement does begin to fail with CPAs and exo-waves, but I figure I need my mobile fleet anyways for these, so I don't care. With mobile fleet support, this setup can deal with CPAs and exos just fine, though I typically will lose a planet or two in the process.
This does start getting tricky when I need to defend another "critical" planet, such as a world with a Mk. IV factory on it or something. I usually start taking a few turrets from some of the "inner planets" and move them to that world, and maybe stick a small mobile detachment there too, like a Spire capital ship or something.
In the later game, where waves start getting quite strong, I will leave a portion of my mobile military (if Fallen spire is on, one or two spire capital ships and maybe a plasma siege or two) to help out. But that still gives plenty of mobile military free for offense.
Again, this sort of setup might require serious adjustment or micro for it to work on lvl 9 and up, but this approach has worked very well for me.
