According to the construction timeline
here the final section towards Okehampton from Exeter opened in 1987, with the bypass in 1988 and through to Launceston in 1993.
That sounds about right.
When I started driving (in 1991) the Okehampton bypass was open but beyond there it was a fairly slow run to the next section of dual carriageway around Launceston.
Up until the Okehampton bypass opened it was a slow slog from Whiddon Down, through Sticklepath and down into Okehampton before climbing out the other side and going along the single carriageway section until you hit the next dual bit.
It was always slow and if you were stuck behind an ancient Bedford TK as it crawled up the hills belching out black smoke (which was pretty much every time during the week, although at weekends it’d probably be a Ford Cortina towing a caravan

) you just knew that it was going to take ages!
That road was terrible until the mid 90s and it’s only fairly recently that it’s been decent all the way into Cornwall.
It used to be quicker sometimes to take the A38 to Bodmin from Exeter, but it isn’t now. Although the A30 does get overwhelmed by holiday traffic more and more these days.
I think the railway being reopened will take some traffic away from the A30 between Okehampton and Exeter and although that’s a small percentage of the actual traffic on that section, as the road gets more and more busy (especially through summer) I could see people travelling into Exeter by train rather than driving. Because even though the original A30 runs parallel to the modern dual carriageway, once the main road gets snarled up the old road gets virtually gridlocked ten minutes later!
Just as an aside - Whiddon Down roundabout (at the start of the Okehampton bypass) didn’t disappear until the mid 2000s or so and I was chatting to the two Graham’s that I work with this week about that.
It was known for being the first roundabout you met once you’d left Scotland (or London if you went via the M4/M5 route).
There were always fresh skid marks on the last bit of the road leading up to the actual roundabout from the Exeter direction (and probably not just on the road actually)!