At boot time and besides the OS flash drive these were also connected:
Nov 12 17:28:17 Tower kernel: scsi 1:0:0:0: Direct-Access Kingston DataTraveler 2.0 1.00 PQ: 0 ANSI: 4
...
Nov 12 17:28:17 Tower kernel: scsi 2:0:0:0: Direct-Access JetFlash Transcend 16GB 1100 PQ: 0 ANSI: 6
They were since removed, but they still count until you reboot, or maybe until array re-start.
No need to format then, but if you still want to just read the first couple of posts in the Unassigned Devices plugin thread and you'll find the answer.
It's a problem with the onboard SATA controller, quite common with Ryzen boards, upgrading to latest beta should help (or disabling iommu if not needed).
Yes.
Unlikely to help and it can't fix the data corruption, best to just re-format.
With the array stopped wipe the SSDs with:
blkdiscard /dev/sdX
Then start the array and format the pool
Most times --rebuild-tree doesn't create a lot of lost+found folders, but it will depend on the level of corruption, I would also recommend converting all the remaining reiser drives to xfs since reiser is not recommended for years now.
Glad all is good, this part is strange, never happened before with v6, especially without an apparent reason, maybe v5 related, but I'll keep it in mind to use the other option when possible.