This is very true, but it’s also what makes a ZFS target especially desirable. You cannot make a ZFS target inconsistent just by power-cycling it, all you can do is interrupt the backup run–and thanks to resumable replication, it’ll just pick back up again from where it left off, the next time it’s available.
Honestly, though, a separate always-on device might really be a better idea. It can be very small and very low-power; have a look at the $139 Odroid H4+. You get a quad-core Alder Lake x86 CPU, 4 proper SATA ports, and extremely low power draw (they recommend a 60W power supply, or 133W if you’re using full-size 3.5" drives. CPU is TDP 12W, and the entire system should idle at 5W or less, depending mostly on how many drives you stuff in).
Note that the $139 I mentioned gets you the processor, board, and heatsink. You’ll need to add RAM, whatever drives you want, and a PSU and case. If you want to go with a pair of 2.5" drives, Odroid’s Gamecube-style case would work well: https://www.hardkernel.com/shop/h4-cube-case/
Their 60W power supply, with a UK plug (I think that’s where you said this would be going?) is $10: https://www.hardkernel.com/shop/15v-4a-power-supply-uk-plug/ The 130W one is $25, but I’m not sure if they’re currently offering it with UK plugs.
If you’re just doing a pair of 2.5" SSDs, though, I think you’d be perfectly fine with the 60W supply.
would a DC600 ssd cope better with the daily power cycles?
Your concern here isn’t really daily power cycles so much as write endurance. But if you’re doing a dedicated tiny backup appliance, it’s not going to take enough writes to worry about, particularly if you avoid installing a GUI and just run pure CLI–the host OS won’t be writing a bunch of ephemeral garbage all the time, all you’ll have is your one backup run per day, and with that being presumably ZFS based… like I said, unlikely to put a big dent in the endurance.
A DC600M is rated for 1.0 DWPD (full capacity of the drive written per day over five years). Prosumer SSDs are usually rated at about 0.6 DWPD, and cheap consumer crap is usually 0.2-0.3 DWPD. The “cheaper” the drive you’re buying, the more you need to go overboard in total capacity to keep it from turning into garbage faster than you want it to.
Also be aware: those ratings are about how confident the manufacturer is that the drive will be USABLE after five years of that many drive writes per day… they do not and should not be intended to guarantee OEM performance after that many writes. IME, most drives are down to about 1/2 to 2/3 their original performance by the time they hit half their rated write endurance. Many of the consumer drives are damn near unusable by the time they hit half of their rated endurance, again IMO, IME.
Your two opposing factors here are 1. it’s just a backup target, so much MUCH fewer writes than the source it’s backing up will experience while CREATING that data to be backed up. But, 2. it’s gonna be on the other side of a rather large pond. Choose wisely! 