I think I’d stick to the original plan and see what the weather/ conditions bring on the day.
With an early start, running up the Molesworth and with the bulk of the distance behind you. there’s always a ~1.5hr return trip up Mt. Altimarloch (you’re coming past the front-gate while on the Molesworth).
If cloudy up top, keep going towards the coast, cross SH1 and spend an extra 30-45 mins around the Redwood Pass and a few beach access points.
Or get off the north end of the Molesworth and use Taylor Pass into the south of Blennie (about the same time as using the main roads, but mostly gravel).
Or add another 1-1.5hrs to the main-road option from Blennie->Picton via the Port Underwood option.
I’d get the bulk of the distance off my chart, then start playing with the remaining time.