This sounds more like a hardware problem than a software problem.
When did you last use a lens cleaner on your drive? It builds up gunk even when unused.
Toast used to be extremely tolerant of drive errors, it just took longer when it found them.
It's doing the conversion first, its not even got as far as writing to the DVD drive yet.
Just tried to burn a m4v file to DVD with Toast 12, 9 hours later still at 35%, on a Mac Pro.
Worst piece of garbage I've ever used, anything else that might be of use ?
BURN still works in Sierra..surprisingly (10.12.6).. But i gave up on this as well. not by the "temp" mpg it creates in folder, I didn't have an issue with that, but by the limits that BURN could support, as there was too many problems with getting ac3 audio over. Even Perian installed didn't fix it.
I put up with that for a while, but then it got too hard needing to install other things to support it.
For "Data" discs, it works perfect though.
Toast always worked in these types of cases, but the slowest just drew me away from that..
encoding small videos with toast is fine, but never 2 hour movie. Took about an hour just to encode. on the "best quality setting"