kernel_task and mdworker use all available RAM memory
First of all, kernel_task gets higher (if I not wrong) because of the mdworker.

Talking now about the mdworker. Maybe the problem is not on your SSD at all. In this article recommends you to turn off Spotlight all devices. I recommend you to test instead with some folders of the SSD you might think Spotlight could get stuck in (large number of files, symlinks, ...). Also, you could try avoiding Spotlight accessing your external disks. Test until you get rid of the problem. Maybe you could find where the problem comes.

Also, you could try mdworker process always at 100%, even when not indexing this answer. Here founds that the problem resides in a Spotlight plugin. That could also be.