They started playing more Tampa 2 in 2019 back with Tedric Thompson. They showed some cover 2 looks last year, it doesn't really matter honestly they are all softzones including their Cover 3. They are one of the most vanilla defenses going in the NFL right now.
Not all Cover 3's are the same either. Dan Quinn liked to press his corners when playing cover 3, play bump and run, and they would play them man on man unless they vacated the zone. Pete has since designed it to where the DBs have to stay on top and surrender everything underneath, and I mean EVERYTHING playing extremely soft.
Jamal Adams entered the equation, and it has mucked things up further. To get the best out of Jamal Adams you have to design a lot of exotic blitz packages creating confusion and chaos. This is not Pete's expertise, or Ken's for that matter.
The best they can come up with is walking Bobby Wagner up into the A gap, or dropping the DE's into coverage for a zoneblitz, 1980's style.
The front has changed this year to a 5-2 and it is the correct decision. To stop the outside zone running game that both the 49ers and Rams employ (the Seahawks as well), you need that extra defender to set the edge running to the outside, and you need power up inside to stop the cutbacks and inside zone runs. That is why the Seahawks play two 1 Tech's predominantly on the inside. They've matched their front perfectly for the division. But what is spooky is the Vikings also happen to run the same system, (It's a popular system.) and the Seahawks got embarrassed surrendering a 140 yards to a backup runningback. They are a mess, can't stop the run, can't stop the pass, getting dog-walked up the field.
Their main problems stem from their lack of masking coverages, playing too soft on the backend, and trying to be a blitzing team to help Jamal be more involved. An upgrade at Corner would only help minimally. They have major coaching issues on defense, and they have fallen way behind the rest of the league in what they are doing on that side of the ball, thus the last place ranking.