Yikes drakecohen. First, I agree with previous responses:
1. Lawyer up
2. Get a clear understanding of your professional obligations based on your credentials and memberships.
While getting stiffed sucks (I still refuse to patronize businesses who stiffed me decades ago), you've got a business operation problem. You allowed this client to be 2 years in arrears. $30k is some serious money - that would keep me in beer and diesel for 20 years!
I would immediately be sure you have written service agreements in place for ALL engagements. If long-time clients resist, explain that your E&O carrier requires them.
At the same time, rethink your billing practices - start billing quarterly in advance or at least get 1/2 when the year end census request goes out and spell it out in the service agreement.
As far as your deadbeat client, focus on recovering as much as possible and also prepare to walk away from that block of business. They left your circle of trust when they stopped paying you. If you want to try pulling their clients that you serviced , avail yourself of legal advice and don't run afoul of professional ethics/standards.
None of the above will be as emotionally satisfying as the 5 actions you listed. If you need to drain some venom, visit the shooting range or rent an excavator for the weekend.