Как насчет этого в начале вашей темы?
if (Thread.currentThread().getName().split("-").length > 1) {
String threadName = Thread.currentThread().getName();
threadName = "thread" + threadName.split("-")[3];
Thread.currentThread().setName(threadName);
}
Тогда в вашем logback.xml:
[%-8thread] %msg%n
... и вы получите что-то вроде этого:
[main ] Process started
[thread1 ] Thread started
[thread2 ] Thread started
[thread1 ] Thread ended
[thread1 ] Thread started
[thread2 ] Thread ended
[thread2 ] Thread started
[thread1 ] Thread ended
[thread2 ] Thread ended
[thread3 ] Thread ended
[thread4 ] Thread ended
[thread5 ] Thread ended
[main ] Process ended