Hello Noah,
Thanks for going Pro and welcome to the forums!
Wow! Such a cool question! It really shows how varied our calendar is being used which makes us proud!
I’m sorry to say though that there is no easy one-click solution for that at the moment.
What you can do is:
- set up an event recurring every 3rd Thursday
- check and exclude (with rule(s)) manually those ones which are not good
- add manually (with rule(s)) when it is the 2nd Thursday
I checked the calendar and in 2017 there are only 2 occasions like that, in September and in December. (Need to look for the months that start on a Friday, right? 😉 ) In 2018 it is only one month, June.
The setup for 2017 would be something like this:

I hope this helps. Let me know if there’s anything else.
Cheers,
Andras