I've been using a raid1 btrfs pool to store offline backups for around 10 years. It's 4 rotating drives (2x4TB+2x12TB). I replaced / rebalanced 3 disks with larger / newer ones already (went fine). I identified a bad usb/sata controller, and lots of bitrots on one old disk (scrub was able to correct a few thousands errors).

I'm getting around 80MB/s read/write throughput (not great but OK for offline backup). I'm able to mount it on low-powered / low-memory devices (not the case for ZFS). Scrub takes around 2 days IIRC (for around 10TB of actual data), so I run it once a year.

I keep it simple and thus am not using advanced features (dedup / encryption / snapshots / subvolumes / raid5/6/10). So far its a good match for my needs.

Don't use any btrfs raid levels besides mirroring. There are long-standing raid bugs that they wontfix in 5 and 6 that have led to data loss.

If rebuild time is truly your main concern, you could configure the 4 drives as raid 1+0. On truenas I think it would be called a pool of 2 mirror vdevs. It would give you much faster rebuild time and still have 1-2 drive failure tolerance.

RAID is not a backup.

RAID is not for data safety.

RAID is for:

1. Ensuring availability of data in the face of hardware failure. That means your files don't disappear when a drive dies and you have some time to swap out for functional hardware and restore redundancy.
2. Presenting multiple drives as one larger unit. This is what striping does, and to a lesser extent the parity-mode levels.
3. Improving performance (sometimes). A RAID mirror is generally much faster to read from than any individual drive because reads can be interleaved across drive members. A stripe can be much faster because writes are distributed across drive members. This is less of a bonus today with solid state/nvme drives, but it's still applicable to spinning rust.

If your concern is protecting your data, set up a 3-2-1 backup strategy.

If you're storing, then offline, read only, checksummed and preferably encrypted. And above all, several tested backups. I'm partial to making erofs images from dirs, but any archive that fits the content type works.

For the random access cache on top, whatever.

Filesystem doesn't really matter once you have a reliable, redundant off-site backup and recovery plan set up and tested.

Really, use what fs feels best for you. And do your backups.

Did I mention backups are important?

I was originally going to use for my important documents

Not quite what you're asking, but if your concern is avoiding data loss, if you haven't already, I'd set up a backup before I started setting up a RAID or similar setup.

What about 2 mirrored pools of 2 drives each, then back up the main pool to the other with either ZFS snapshots or a tool like Restic.

Ideally you also need an offsite backup of important files too, but that gets you part way to a robust system that can handle corruption or accidental deletions.

While redundancy in a drive setup helps, it’s not really a backup and thus not a “safe” way to store important information on it’s own.

That said, selecting the way you setup a raid system is based on risk and utility. I have a raid1 with a hotspare for important files. And use raid5 with 3&4 drives for less important stuff. You can also optimise for reading speed for example (as the same file can be drawn from multiple drives)

I'd stay on zfs, I simply don't trust btrfs’ raid implementation. For very important documents I also set copies=2 (or 3) on that dataset, just in case.

And as others already said: 3-2-1 backups ;)

For your situation I would be more likely to go with a single drive with btrfs and dup for metadata redundancy. Regular snapshots and scrubs.

Use a second drive in the same system with btrfs to store snapshots at wider scheduled intervals. These will be bigger since no CoW on the separate file system. Scheduled scrub here too.

Use a third drive with ext4 as a backup target using a separate backup mechanism.

Use the fourth drive as a spare, or in a separate location as a target to send the backups if you don't already have an off-site solution.

Acronyms, initialisms, abbreviations, contractions, and other phrases which expand to something larger, that I've seen in this thread:

6 acronyms in this thread; the most compressed thread commented on today has 15 acronyms.

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Maybe you could switch to a raid10 (mirrored striped vdevs) for faster rebuild time.

BTRFS is relatively similar to ZFS when it comes to their raid implementation, though using raid5 or raid6 comes with some caveats.