What are the specs on that 3d printer? Is there heat involved?
If it's at all like my B/W laser, the last thing in the world you want to do is to put that on a UPS.
When my printer starts up (not plugged in to UPS), my UPS beeps just because of the momentary current surge causing a voltage dip.
It's way more likely to ruin the files by accidental deletion, destructive commands, software errors, massive hardware failure like your power supply failing and destroying all your drives at the same time or your house burning down.
Yes. This is important and shouldn't be overlooked.
I am confused as to what Western Digital's long-term goals are.
Their long term goal is to stay in business to make money for their C-suite, their board of directors, and their stockholders.
E.g. 47 years ago, long before consumer HDDs, long before SSDs, Western Digital had a product that "revolutionized" storage. It was a chip that let you read and write "floppy disks".
A floppy disk was an 8" diameter flexible, removable, rotating magnetic storage media that stored about 240 Kilobytes of data.
I don't think WD will still be around 47 years from now, but you never know.
Since you're planning on mirroring, there's one obvious solution: buy one of each, WD and Seagate.
That way you're not screwed if both drives of the same lot have the same design flaw or manufacturing defect and both fail at about the same time. In the past this has happened quite a bit.
Having only two drives means the Synology shouldn't be overly loud. If it's still too loud for you, search the /r/Synology subreddit. There are ways of quieting a box. Here are two that come to mind:
Make sure the Synology is on a very thick, solid surface. E.g. a hardwood table. Not some cheap particle board thing. Add a thin mat underneath if necessary (e.g. like a mouse pad material).
Add some velcro-like material in the right spots.
Here are two older posts from that sub to help you get started with your search:
https://old.reddit.com/r/synology/comments/klt119/how_to_reducedampen_resonating_vibration_sounds/
https://old.reddit.com/r/synology/comments/175fv1x/holy_crap_night_and_day_difference_vibration/