I also use DVD-R, not only DVD-R as you only make a backup this way every so often. Sorry for the long reply.
I think the frequency is up to you as you take into account your personal risk/benefit. I must admit my workflow requires several tedious steps, which makes it a proces that I perform less often than I want to. I’m thinking about automating parts of it, as that enhances the chance of committing to it.
You can simply backup from DT into a ZIP as long as the zipped DB is smaller than the disk size and a tiny bit of overhead. Either use optical disks with a large capacity like the more expensive Blueray if your DB is large, or create multiple databases with a certain size.
There are several ways to handle the backup process I guess. One way is to create an (encrypted) disk image with disk utility. Then mount that image and have DT write the zip to the image. Then insert the DVD-R and drag the image to the disk and hit burn. The software is incorporated in macOS, at least when you use a Superdrive. I haven’t used other drives, so those might require additional software.
Steps to automate might be to automatically copy the DB to a temporary location, give it a unique name, zip it, empty the disk image once zipped, write the files to the disk image and create a SHA checksum. When you’re ready to backup you only have to drag the image to the disk after you loaded a disk.
After burning I test the backup simply by unmounting an remounting the drive to decrypt the content on the disk (and if I’m motivated enough copying it, compare the checksum, open it in DT and view some files )
This might appear as a huge undertaking, but it’s doable. You can of course minimize parts of it to your liking.
As a side note: DT might benefit from a ‘build in’ checksum to compare databases. E.g. when creating an archive. When you copy the archive from a different medium and reopen it in DT you could have DT automatically check it’s integrity based on the checksum in the archive. Would that be something to incorporate @BLUEFROG? This would also require a ‘restore archive’ function (basically nothing more than copy the ZIP, unzipping the content, checking the checksum). Checksum generators like SHA256 are a standard part of macOS.