The delay is unusual, both because Jim has historically been very quick and because of how optimistic he's been about finishing. At Dragon Con 2017 he was forecasting that Peace Talks would be done by the end of 2017 ("six to eight weeks after finishing Brief Cases") and in the Brief Cases Reddit AMA this past June he was aiming to finish Peace Talks before his wedding this autumn. So it doesn't seem to be like the Ghost Story delay, where the issue was artistic rather than logistical...
Ah, this thread inspired me to check the Jim-Butcher.com Upcoming Works page, where just yesterday they added a "Why’s Peace Talks Taking So Long?" explanation, which does go all the way back to the divorce and The Death Of Frost in 2015. (I believe this is the first 2018 update of that upcoming works page.) They have also posted a new DF short story, Christmas Eve, as a gift for fans.
Anyway, I remain optimistic that future waits will not be so long now that these issues have been worked through. Jim was a marvel when alternating between Codex Alera and Dresden Files volumes, so hopefully something similar will develop with Cinder Spires. I'm not aware of any other novel besides The Olympian Affair planned in between Peace Talks and Mirror, Mirror, but I haven't been keeping very close watch.