You might want to look up SMR vs CMR, and why it matters for NASes. The gist is that cheaper drives are SMR, which work fine mostly, but can time out during certain operations, like a ZFS rebuild after a drive failure.
Sorry don’t remember the details, just the conclusion that’s it’s safer to stay away from SMR for any kind of software RAID
EDIT: also, there was the SMR scandal a few years ago where WD quietly changed their bigger volume WD Red (“NAS”) drives to SMR without mentioning it anywhere in the speccs. Obviously a lot of people were not happy to find that their “NAS” branded hard drives were made with a technology that was not suitable for NAS workload. From memory i think it was discovered when someone investigated why their ZFS rebuild kept failing on their new drive.
Instead of one super chunky battery, how about a laptop with replaceable batteries, in combination with a UPS?
UPS is so you can actually replace the laptop battery with a spare one , even during a power outage. Just run the laptop on AC from the UPS while changing batteries. Or see if you can find a UPS with a long lasting battery. Entry level ones only have like 15-30 minutes of battery life though, since they’re more intended for safe shutdowns or brownouts.