Sequence object blocking
The other 700+ sessions were waiting on LATCH_EX....We recently changed the CACHE value for this sequence from the DEFAULT (50) to 200.

If you've got 700 waiting sessions, my first thought is that the cache needs to be much higher given the workload. I would recommend trying something in the range of 500-1000 (and maybe keep increasing) until you see the waits either stabilize or drop off. There should be a sweet spot you can find with a larger cache value.