I think we’ve exhausted most possibilities. I looked at the DD-WRT forum and the X6/R8000 thread has a very large number of posts, and clearly the builds have had quite a few issues in the past, including with repeater-bridge mode.
We know that AC-only mode isn’t working on your X6 in repeater-bridge mode and causes a large number of wifi driver error messages in the system journal. These error messages seem to be specific to your set-up (X6 running DD-WRT) so, as I earlier said, the router seems to be sending some data value(s) and/or sequence(s) that the driver isn’t able to handle.
It seems that the problem is either with AC itself on the router or with AC in repeater-bridge mode. My own hunch is that it’s the latter, but either way it doesn’t seem to be working with your particular set-up.
Unfortunately, I’m unable to offer any further suggestions, since the root cause seems to be an incompatibility between the OSMC dongle’s wifi driver and the DD-WRT build for the X6.