Probably easiest to use Paisley Road West until just before Ibrox and turn onto the A8 (Shieldhall Road) past the front of the stadium and follow that all the way through Renfrew until you hit the A726 and turn right for the bridge. It's pretty direct and the only really busy bit is the roundabout that takes you into the Braehead shops. Saves you having to negotiate the one-way system in Paisley.
Even better ride to the Erskine bridge exit of the path to Dumbarton, cross the road to the path to over the Erskine bridge, exit and cross to the garden centre, then access the Mar Hall from the Hospice road.
Need to scout the last bit, but I saw a bike path leading to Mar Hall from there on Google maps.
No busy roads to battle on.
Our own
@Rasmus almost got flattened on that Braehead roundabout, on a quiet morning too!
Btw, there's a back way to Braehead, only for bikes, but I can't remember the exact route, been only once. Passes by a pond.