It's not being detected by Linux, so there might be a driver, if you can get it detected, it will never work if it's not, and this is not a software problem.
btrfs is not really standard RAID.
You can add (and remove) disks at any time, you can even uses different size disks.
But note that btrfs raid5/6 is not yest considered safe for production use, it can be used, I have several raid5 pools, but I only recommend them for anyone with stable hardware and good backups.
Any LSI with a SAS2008/2308/3008/3408 chipset in IT mode, e.g., 9201-8i, 9211-8i, 9207-8i, 9300-8i, 9400-8i, etc and clones, like the Dell H200/H310 and IBM M1015, these latter ones need to be crossflashed.
Next beta is going to align the partition on SSDs on the 1MiB boundary, like Windows does, this should improve performance, try that that once it's available, you'll need to reformat the cache though.
Overclocked RAM is a know issue with Ryzen, see here, also check the power supply idle control setting, even if before you were overclocking you should stop.
Main difference it that on the array you have individual data drives each with its own filesystem protected by one or two parity disks, each pool is one btrfs filesystem that can be or not redundant, you can use any of the available btrfs profiles, depending on the number of devices.
Yes, this is an old issue, not related to this release, it appears to be a fuse issue that happens when a file is removed and then moved, I believe it happens mostly when using for example a media processing docker.
Now it wouldn't be able to sync 100% because of the read errors.
Correct, parity needs all others disks except one to recover, so a known bad disk should be replaced ASAP or if another one fails you'll be in trouble.
Almost certainly unrelated, if there's nothing logged and it started doing it out of the blue it's most likely a hardware issue, though you can still try safe model without VMs/dockers to rule that out first.
I'm not seeing any disk read errors, there are some ATA errors on disk7, and since the disk look healthy it could be the typical SASLP problem, parity was updated after some of those, possibly incorrectly, you can swap that disk with another one using the onboard SATA ports and run another check, also I would recommend replacing the SASLP with an LSI ASAP.