I trust MB SATA more in terms of reliability. HBAs tend to overheat too.
However, if RAID topology allows, I'd try to spread the drives such that either one of MB or HBA failing completely would not bring the array down (RAID10 with 1 HBA, or RAID5/6 with 2 HBAs).
That's expected for 4k synchronous writes. They need to complete one by one and you're writing to a hard drive.
If the drives are mounted firmly then it should be fine with proper outside packaging.
New > Used > Recertified > Refurbished
WD > Toshiba > Seagate
Based on that, no I would not.
Anything is fine as long as it's CMR. Always check the datasheet thoroughly.
This is a refurbished Seagate Exos, so if you actually do get 5 years of warranty that's a good deal
This site oscillates somewhere between useless and malicious. Some of the worst scams, on amazon, of all things.
total commander -> ctrl+b to flatten the view of directories
Disregard the advice about SSDs. Start with 1 NVMe for boot and only add more if you have a need. Either for running VMs or if additional cache is actually needed.