Mysql master master with high latency

We have a single master setup with many slaves in germany and some slaves in australia.

To have faster writes in australia we want a master-master setup between germany and australia. Read slaves will remain under the local master in both regions.

We tested Perconas Galera, but the commits are synchronous, and thats why writes are way too slow.

Is there an option in galera to speed things up? Maybe tranfer multiple transactions in one block between the two masters? Or allow commits to be accepted before the second nodes applied it?

Is there another master-master solution, that works better (maybe asynchronously) under the given high latency of ~300ms?