From the release notes:
It's always a possibility when the driver is not included with the kernel, nothing LT can do about it but wait for a newer driver.
You can also take a look here since it might take me longer to reply due to weekend:
https://forums.unraid.net/topic/46802-faq-for-unraid-v6/?do=findComment&comment=700582
That's normal with different capacity devices in the same pool, free space should be correct though.
Also note that cache2 is not currently part of the pool, despite the GUI showing it is.
Very possible to get 10MB/s writes with SMAR drives, I get that with some of mine when they hit the SMR wall, you should at least avoid SMR for parity since that will impact writes to any array device, even non SMR.
OK, but the array won't auto-start with a missing disk, you can start it manually if you click the checkbox next to array start, but still need to remove the disk from the array, even if it's empty, or the array will be unprotected.