That failure rate is well above normal. I’d start considering other causes. For example, are you sure that the power supply is good? Have you played with spin down or similar variables causing excessive wear? Etc.
43*C is high in my book but OEMs now specify allowable drive temperatures as high as 60-70*C IIRC - likely leading to premature drive failure but from their perspective that’s ok, as long as it happens outside the warranty period. These temps are easily reached in cheapie external cases without active cooling during a big backup.
I doubt WD will tell you anything but to pound sand. However, your failure rates are so egregious that they may feel some mercy, so I’d contact them and see if they can do something even though your drives are out of warranty.
FWIW, this is why I don’t take the backblaze data as gospel. Their drives are running at 17-33*C, they are automatically replaced after four years, etc. which likely is a different use case than what we have spinning at home. Many of us run our pool hardware a lot longer than 4 years!
speaking of hardware, consider getting a better case. My Lian li a75 keeps the drives at 5*C above ambient. It can hold up to 12 drives and I simply staggered mine to leave an empty slot for every two drives. Three 120mm fans do the work, a dedicated Fan controller with feedback thermostat controls their speed.
my NAS drives are helium HGSTs that I bought used, most were 2 years old. One drive was DOA out of the box, another failed two years in, both elicited a full refund from the seller (3-year warranty). At this point, the remaining drives are 4-5 years old. No issues since. I have a cold spare or two handy.
One benefit of buying used is that you’re unlikely to get drives from the same batch, so failures should be more random.
lastly, I do not understand the benefit of a hot spare in a home setting. Buy the drive, qualify it (bad blocks, etc), pull it, set it aside. Why incur wear and tear + power consumption when you don’t need to? I get using hot spares at a data center or remote backup where physical access is limited but in a home setting it makes no sense.