The coastal route joins the central route at or near São Pedro de Rates. From there, one route heads to Valença, and into Spain at Tui.
In my experience, the first three day or so, from Porto, along the coast and swinging northeast towards Rates are beautiful and easy walking. The people are wonderful and English is widely spoken. However, there are very few pilgrims.
Arriving across the bridge from Portugal, in Tui, you are at one of those "magical" just over 100 Km Camino points that qualify for a Compostela from the Pilgrim Office for having done a Camino if you walk the "final" 100 Km to qualify. This is analogous to Sarria on the
Camino Frances.
This established, once at Tui, and especially in season (after Easter) the pilgrim numbers will increase greatly. At Tui, folks who are not able, do not have the time or inclination to walk a longer Camino will generally start from the Tui Cathedral. It is a good place to stay overnight and to start from.
If you are walking over the girder bridge (designed by Gustave Eiffel's firm) from Valença, remember to change your watch AHEAD one hour. Portugal is in the same time zone as London, UK (UTC +1 during the summer). But, Tui and all of Spain is in the Central European Time Zone (UTC +2 in summer).
Walking from Tui towards Redondela, you pass through Porrino. The approach to Porrino from the south is through an industrial area of auto assembly light factories, warehousing, etc. It is several kilometers of straight, somewhat soul sapping (IMHO) sidewalk hiking. There is precious little shade to be had and lots of trailer trucks.
There IS a pleasant, more off the main road alternative after Tui. It is written about elsewhere in the Forum. Search for "Porrino." Be aware that local interest conspire continually to vandalize, deface, or move yellow arrows and other directional or informational signage placed to provide this more relaxing alternative to you. Do your research.
I made a wrong turn in 2015 and became "lost" in a forest for nearly two hours. The tree canopy was thick enough that I could not see the sun. I eventually found my way, but onto the industrial route...Yuk! I will try to do better this year.
I hope this helps.