Ok, so ignoring the slow potential write issues, which as i've said likely will not affect me... And with the solution above of using "SSD Optimizer" shoudl negate that if it happens anyway.
Again, fine, yep slow write speeds once the buffer/cache/whatever is used up. No problem at all, it's not being used in the middle of the night so it can sit there and re-write whatever it pleases in the background. My data usage will still not go near the rated 180TB in a year. I'd be lucky to do 20% of that. Read speeds are unchanged here, so no problems so long as I have something like SSD Optimizer to ensure the initial speeds are good (for example if I dragged 100GB off a USB drive, I wouldn't want it to slow down to crawling speed whilst USB3 still has tonnes of bandwidth available).
As for the OS issue, I am not sure you're correct here. From what I had read it sounded like the major reason the Seagate Archive 8TB had such slow write speeds is because it DOESNT use the OS for processing. They are drive managed SMR, meaning you can use them with any OS, and similarly any enclosure. After all, that's what the Seagate 8TB external drive is, the archive drive just plonked inside an enclosure with a power/usb connector.
I am not trying to discredit everything you say, I am considering it. It's just, you come across very paranoid in the way you present your information. It makes it difficult for me to put much faith into it, just because it has an air of "arghhhh the world is ending if we use SMR".
Western Digital Reds in the 6TB variant are even more expensive here, the 3TB being the cheapest (about $150-160 per drive). But with only 8x3.5" bays I dont want to use anything less than 4TB drives to ensure I have enough room down the line should I ever get a half decent internet connection.
Can appreciate the major con though, that these are early drives. And quite often they can be a bit "meh". Mmm lots to think about, unless a really nice sale happens...