Hi, welcome to the forums!
What is the goal of the increased longevity? The break-or-live point for hard disks is generally spin-up, it puts more stress on the system than steady spinning. Things that would happily spin for many months, can break in a peak like that.
Manufacturers don’t test or advertise their consumer grade HDD’s for continuous usage, still I think that is the easiest on the hardware.
In case of your external HDD, the main worry would be the temperature in the enclosure. If you have a way to keep the internals at room temperature, in a dry environment, the HDD would be happiest.
Now, getting back to the goal behind longevity.
So, it is not “long term storage of tax papers and baby’s photos”, but “having convenient access to my (replaceable) entertainment”.
It would be a pity if you could not swap your book halfway a holiday because the HDD died, but not the end of the world.
If that is the case, longevity could have a financial element. When the disk dies, you need another one. If start/stop reduces lifetime, continuous spinning increases power usage. More power means less money for new disks.
A laptop-HDD takes less than 2W for reading, a regular 3.5 inch HDD takes about 5W for reading (just spinning should be less). Here a kWh costs about 0,3 Euro, so a bit more than 2.5 Euro per Watt per year. Having a 3.5 inch HDD on a 230V → 12V power supply will have some conversion loss, but most of the time the disk will be idle. That would mean about 15 Euro per year when not spinning down.
If you can spin down the disk quickly and have it inactive most of the time, it should be less than 5 Euro per year (power spike for spin up, reading, spin down; power supply should be <1W at idle to conform to EU standards)
So, now it depends on your backup strategy and the cost of a new HDD whether you should look at spinning down or keeping the disk active. The difference, at 0.3 Euro/kWh, is about 10 Euro/year. I have no idea how long a sometimes-active-external-HDD will last, or how long it will last when constantly spinning at not too high temperatures (for those enclosures it is often cosmetics over function).
My external HDD’s are either chucked and now internal, or disconnected most of the time. I would expect them to still be running 10 years from now, so it would mean 100 Euro price difference for spin down vs keep spinning scenario. The spin down scenario means that 5 years from now I can spend 50 euro on an even more power efficient SSD, then keep the HDD in ‘cold storage’ and use the SSD instead. Let us know in 2027!
Sorry for the long story without clear conclusion 