Switching ext4 rootfs as data=journal mode

I don’t know if I can answer, but is the SSD’s partition in “/etc/fstab”? During a mount the fourth field specifies options. If your ext4 partition has a journal available (and it looks like it does), then the async option in field four probably does what you want. The “defaults” parameter is how most people would set permissions, and this is equivalent to:
(field four is a comma-delimited list of options)

Note that a journal is not always what people think it is. One reference:

The short answer is that the journal will cause a non-fatal loss of data which has begun write, but which has not completely flushed to persistent memory. If the journal size is exceeded, then the entire filesystem can be lost. In all cases though you will have some loss of data if there is a write started, but not yet complete when power is lost. Synchronous mode is the only guaranteed method of not corrupting and not losing data, but performance will suffer horribly, and any solid state device will fail far sooner than normal.

So far as I know the default is to mount with the use of journal if the kernel supports the journal feature and if the filesystem supports the journal, except when told to not use the journal. I can’t guarantee that, but I guess a question is what prompts this question? Was it loss of data? Was it complete corruption and loss of the partition? Or is this just research on the topic?