Best way to add a 240v 30A circuit
Your easiest bet will be a cross between 1 and 3
Your easiest and simplest bet is a cross between approach 1 and approach 3, using a subpanel with a breaker interlock between the two breakers in it to keep you from overloading the feeder by running both kilns at once.  The good news is Siemens, Square-D QO, and Eaton all have fairly nice interlock kits available now, and for decent prices too, while a basic subpanel (you could get away with a 4, 6, or 8 space unit here since you aren't going to want to feed general load from this panel) isn't expensive either.