the fact is I can get for about the same price a server with 4x 6TB WD Ultrastar SAS HDD (plus full RMA and warranty). I would feel safer with enterprise-grade SAS HDD than consumer slow/low power laptop drives. As Calomel puts it (<
https://calomel.org/zfs_raid_speed_capacity.html>) there is no speed benefit from SAS vs. SATA, it's mostly a reliability choice.
I like it! Matter-of-fact, my home server is somewhat similar, except I have even less redundancy: I have two enterprise near line 3.5" drives. They are quite old by now (about 3 and 5 years), but they were reasonably current capacity when I bought them.
Just one warning: About 15 years ago, everyone "knew" that SCSI=SAS drives are enterprise grade, and IDE=SATA drives are consumer grade; my colleague Erik Riedel even published a nice research paper about that. With enterprise nearline drives today that is no longer true. You can buy the same drive model with either SAS or SATA interface, and it has the same guts, the same quality, the same performance, the same reliability. If using SAS costs you extra, don't bother.
WARNING: I did not say that in general all SATA drives are as good as all SAS drives. At the low end, mobile and consumer drives only exist in SATA; and high-end SSDs only exist in SAS. But in the middle, in the nearline segment, most drives are available with either interface.
... I would setup a raidz2 pool, which is safe enough considering I've also got remote backups.
THIS! You nailed the argument. Data dependability or survival depends on many factors, and the first-level RAID redundancy is only one of them. If all you have to guarantee survival is an unmanaged RAID, then single-fault tolerance is not enough today for high-value data (where loss of data would have significant consequences, loss of money or loss of lots of time). But 2-fault tolerant RAID is probably sufficient for small systems, with a handful disks. The moment you go to thousands or millions of disks (such systems do exist), you probably want at least 3-fault tolerant RAID. But that is all assuming that you ONLY rely on RAID. To begin with, you can make a disk system considerably more reliable by running the disks are reasonable and stable temperatures (which requires monitoring the temperatures, and controlling fans and/or climate control). Scrubbing data also helps significantly, because it finds latent sector errors earlier, and reduces the time window where latent sector errors can metastasize. Monitoring disk health (with SMART) is also good, because you want to replace a failing disk while it is still readable. All these things make it more likely that a "only 2-fault-tolerant" system will survive.
And if it doesn't survive, that's when good backups come in. For example, I have only a 1-fault tolerant RAID at home (2-disk mirror), but I have the first backup that's never more than 1 hour out of date right at home, on a disk that is in a fire- and burglar-protected safe at very constant temperature. The second backup is about a week out of date on average, but sits in a different building many miles away. With that combination I have 4 copies of the data, in 3 different locations, and that's good enough for my home stuff. If I really cared about my data (like it was worth a lot of money), I would have an extra copy on a cloud provider I trust, but for now that's too much hassle.