I have a TrueNAS box that is the main component of my home lab (runs several  VMs and apps in k8s). There are about 20TB of files on there. The bulk of the files by way of size are photos.

I currently back that up to another TrueNAS box that exists solely for the purpose of backups. I'd like to replace that box with a low power/heat machine that I can put in the safe.

The problem is that that second box probably wouldn't run ZFS, and if it did, it probably wouldn't support ECC memory.

Suppose a bit gets flipped on a file on the backup box, I'm assuming there's no feasible way for the primary NAS to catch that and re-write the file? As I understand it, tools like rsync just use the filename and size of the file to determine if a file is "the same," and a file with a single flipped bit wouldn't be identified as being different....

I'm in exactly that situation.

I've got a Synology NAS I'm using to back up a system running ZFS.

I've got the NAS providing an iSCSI target that is used as a ZFS pool on the system and I 'zfs send | zfs recv' entirely locally.

It feels dumb but it works.

You can "zfs send" your dataset to a tarball - it's not optimal for a variety of reasons but you can do it

You can run a pretty low power TrueNAS box, but I don't think you're going to get a low-enough to run inside a completely closed safe
At minimum you'll need to get low power ssd and ram instead of spinning disks - 2x 3.5" hdd will generate enough heat to need ventilation, and some ssd controllers will also need ventilation

If you're trying to do this as a major disaster recovery thing, consider instead/also backing up to an offsite bulk cloud storage like backblaze b2 or aws s3 glacier
A tool such as rclone can encrypt locally before uploading and the cost should be around $30/mo for 20TB on B2, and cheaper for S3 Glacier (but you'll pay alot more to get your data back out than B2)
Both services have a way to do a one-time bulk transfer with a standalone device if you don't have enough internet to upload the 20TB initially

Does the Synology NAS have ecc memory?

Backblaze and others would be pretty expensive... For instance, backing up 20TB to something like Wasabi would be $140/month....

Your understanding is incorrect. rsync is a block-level copy. If a file changes at all it gets seen as different compared to the destination and is copied over. It doesn't validate what makes it different, just that the blocks are different. Changing a single bit or byte on the file will change the block(s) the file is on and it will get flagged by an rsync.

Some do.  DS 723,923,1522 at least do, and those are the size of what you'd store in a safe.

ETA: And those units can be upgraded to 10GB ethernet, which is helpful if your snapshot replications are on the ample side.

OP, you can run ZFS on any type of Linux/Unix box.  And while ECC is recommended while running a ZFS file system, its not mandatory.  My file server in my basement is running Debian 12, and has a 34 terabyte ZFS volume on it.  System does not use ECC memory either.