Unfortunately, the equipment needed to produce stamped DVDs costs a small fortune. That option is pretty much out. And, as you say, the burned ones have lousy data retention over time. Careful storage and good quality blanks can help a bit; but that just isn't an archival medium.
Hard drives will retain the data considerably better; but their tendency towards occasional but total failures means that you can't really depend on just one.
Tape is the traditional solution to this sort of problem, and continues to be a good idea. Somebody else might know what specific sort of tape setup makes the most sense these days.
One general, philosophical, consideration is that digital backups are more of a process than a one time thing. Most digital media have lousy lifespans compared to older analog ones; but can be copied without error at low cost. A backup strategy that involves keeping good enough backups to last for a certain space of time(say 2 years, could be longer or shorter depending on a number of factors) and, at the end of that space of time, shifting all the data over to a new medium(either a fresh batch of the previous medium, or a batch of one from a newer technology) to guard against obsolescence and breakdown can be useful here. For example, there really aren't any systems available to the individual user that will preserve data for 20 years at any reasonable cost. However, buying a couple of external HDDs now, keeping a copy of the data on each, transferring the data to two of whatever we are using four years from now, and so forth with those copies, 8 years from now, and so on, will do so, without too much expense or hassle.
