There are several reports in the forums of this shfs error causing /mnt/user to go away:
May 14 14:06:42 Tower shfs: shfs: ../lib/fuse.c:1451: unlink_node: Assertion `node->nlookup > 1' failed.
Rebooting will fix it, until it happens again, I remember seeing at least 5 or 6 different users with the same issue in the last couple of months, it was reported here that it's possibly this issue:
https://github.com/libfuse/libfuse/issues/128
Attached diags from latest occurrence.
tower-diagnostics-20200514-1444.zip
There were no errors on the last check so everything if fine for now, only need to worry if you get more errors on a future check (without any unclean shutdown or other issues).
No, v6.8.x uses a different engine, and it should mostly auto-tune, though there are some known issues for very large arrays, usually with 24+ devices.
BX500 is a value SSD without DRAM cache, not good for performance, the MX500 for example performs much better.
https://www.tomshardware.com/reviews/crucial-bx500-ssd,5377.html