You can't hire a really good defensive coordinator because they have jobs and the only way you can get them is to promote them to head coach. Most of the time this turns out to be another exercise of the Peter Principle.
DC's that are available are typically DC's that have been fired because they sucked at the last place they coached. You need a DC that has some NFL experience and had at least some success at the NFL level so your choices are rather slim.
Just gotta roll the dice and see. Or grow your own, which I favor.
See my post on Kris Richard and why this Defensive Coordinator that never had a defense give a team a losing record does not have a job.
Dennis Thurman is at Jackson St. with Deion Sanders as DC. He was Kris Richard mentor with an excellent resume but is not in the NFL as DC. He was an ex-Cowboy and has the only legend of the Cowboys of the 80's in "Thurman's Thieves".
Mike Nolan in the last 10 years has had the NFL worst defense twice (2014 Falcons, and 2020 Dallas Cowboys), and was HC of the NFL worst team in San Francisco with the Big M&M as his OC. Jerry Dumbo GM Jones is about name retreading.
Tell Dave Wannesdat that you need NFL experience and success to be a defensive coordinator. He will show you his Super Bowl ring. You only have to know the LTD. Lead, Teach, and Develop.