Those are the most usual, so no need for diags, if you don't have more issues, especially after some heavy IO like a parity check, it might not be a general problem, which unfortunately will make it harder to fix.
For now I'm mostly interested in someone not having issues with that combo, to see if there's something different, like a specific LSI/drive firmware or even HBA model, but in case your issue was really a connection problem and if it doesn't happen again in the next couple of weeks or so then please PM me your diags at that time.
You can use raid5/6 btrfs pools for better performance, I have several, just be aware that there are still some known issues with btrfs and raid5/6, so it's important to have good backups.
If you have an LSI and 8TB Ironwolf drive(s) (only model ST8000VN004) probably best to stick with v6.8 for now, there have been multiple users with issues that can result in disable disks after upgrading to v6.9, on the other hand if anyone using them upgraded without issues please post back.
Upgrading the firmware is very easy and can be done with Unraid, unfortunately the erase BIOS command doesn't work with the Linux tool, but it's still easy to do if you boot with DOS flash drive, or UEFI shell if the board has UEFI.
You can do that, as long as there's no "all data on this device will be deleted after array start" warning in front of the cache devices Unraid will import the existing pool.