hi, my experience of traffic in Spain is that it's a bit worse than France but still a lot better than the UK. Outside of cities, and on minor roads, volume tends to be similar to France.
also, be aware of Via Verdes
, which might give you some additional options.
the St Jean de Port to Roncinvilles route (as walked) is off-road and quite steep. there's a road (from memory) that you can use to get to the highest point of the pass, and then I think there's a fence/stile to cross and you have some reasonably flat/good off-road track, and then a road again after maybe 1 mile (guessing, it was years ago I was there).