Hi,
I walked last summer with my 13 and 16 year old children from Lisbon to Santiago in 23 walking days, we took one day off in Porto so 24 days over all. We switched from the coastal to the central from Caminha to Tui. I think we could have maybe finished one day sooner for a total of 22 walking days. To stay on the coastal the whole time would mean an average of at least 30km a day for 21 days. I wouldn't enjoy that.
The summer of 2017 we walked from Porto to Santiago in 9 days, switching from the coastal to central after Vila do Conde, we could have maybe done that in 8 days. We took 13 days from Lisbon to Porto. That would be my recommendation for you, the first day on the coastal out of Porto gives you a taste of the rest of the days to Caminha. I haven't walked the coast in Galicia so I don't know what you will miss there. However, the central from Rates to Tui is probably my favorite part of all. I think you could get that done in 21 days from Lisbon assuming no 'bumps' in the road.....
Good luck!!
Buen Camino
MaryEllen