As several people have said, stick to the coast and definitely go up the hills behind Harlech. Or an alternative and easier but safer than the main road is to take the old road up to Harlech, through town and then up steeply the road beside the car park, through the bollards. This road then joins with the original route after a mile. The views are great.
The inland route is also great fun above Trawsfynydd to Dolgellau with two great cafes. One at the new Stiniog site by the old fishing lodge on Trawsfynydd lake and the other at the forestry bike centre purpose built.
Have a great time which ever way you go.
The bridge has been a great pain for us locals, but when its done it will have a proper combined cycle/footpath path and no more tolls