If your average traffic rate exceeds the average disk IO rate that you can flush logs to disk then you will never catch up and logs will backup in memory (unless you are discarding) until you get an out of memory error.
The back pressure from transitioning to sync mode exists because the disk IO rate is insufficient and the only relief is to push back against the source of the overload lest you consume all memory and crash.
It sounds like you need additional disk IO throughput. Consider changing the disk type to a higher performance class or adding additional additional disks to your instance and striping your filesystem. Depending on the IOops your VM is currently pushing vs it’s nominal capacity you may need to consider using an instance type with higher IO throughput.
Fundamentally the IO is not keeping up, it’s not CPU bound and it’s not memory bound, so fix the root cause.