Why not walk from Porto cathedral? Time constraints, or scared off by the bad reputation of the urban path (which isn't too hazardous by any means)
Anyhow, the local metro will take you so far north and is short connection to the camino (Vilar do Pinheiro station), from there you could walk to Rates.
Or catch the train to Barcelos.
Or walk from Porto cathedral to metro stop, like I did, metro back to Porto. Next day, train to where you left off, walk to Rates, stay the night. Then next day, walk to Barcelos.
It's easy enough to work out how the metro line follows the camino with google maps and a
Brierley's/guide map. Thats how i managed it.
Brierley's has a chapter explaining an option.
Oh, and time your visit to Barcelos for a Thursday, as they have a large market then. Also there's a 7pm pilgrim mass.
Included some easy to google info, to get you started. Have fun planning!