What size are your iso files? If they’re around the 7-8GB mark, they’re AVCHD images, i.e. HD video designed to be burned to (and played from) a dual-layer DVD-R disc.
(The AVCHD format uses the same file structure as Blu-rays - hence the BDMV and CERTIFICATE folders - but at a size that means you don’t need a Blu-ray burner or disc. You do, however, need an AVCHD-compatible Blu-ray player to view the result).
That all being the case, your Superdrive should be able to burn those iso files to dual-layer blank DVDs. I don’t know Mac software, though - presumably Toast (or whatever) can open an iso to burn to disc? If it can do that, it doesn’t need to know what the actual content is - it’ll just burn those bits.
Ah! That makes a lot more sense! Ok so I’m on a OSX v10.12.6 using Toast Titanium 11 with the HD plugin. I think most of my problem is deciphering what Toast is asking me! So I need to aim for using a DVD-R in dual layer rather than burning to a BD-RE. No wonder it keeps spitting it out! Thanks, Jonno. I’ll let you know how I get on.