I still think a Camino Portugues fits well into the length of time you have. Of course, it will be hotter then, and a Coastal (or Senda Litoral) Portugues sounds like it would provide more opportunities to cool off, which certainly sounds appealing. I haven't walked the Coastal myself (after Vila do Conde) but a number of reports say that it has a little less of the "Camino feel" than the Central, with the presence of all of the holiday-goers just there to enjoy the beaches. I expect this would be even more so in the summer. Others don't have an issue with this. And if you are there to enjoy the beaches, too, it may not be an issue with you. If you have any questions or concerns about this, I would ask on the Camino Portugues Coastal from Porto sub-forum. Others there have much more experience than I.
Personally, I wouldn't walk a day or two on and then take a day or two off, alternating. If I wanted to take it slow and easy (as I often do) I would just walk shorter stages. I suggest this for two reasons. One is that you get into a certain rhythm walking every day, even if they are just short days. I wouldn't want to break that rhythm repeatedly. The other is that one of the most valuable experiences (for me) on all but the least-walked Camino routes is the community of fellow pilgrims, especially the ones you see day after day because they tend to walk the same distances as you. If you stop for a rest day or two every few days, that will really affect your ability to enjoy that kind of community. As soon as you get to know people you will stop so that they can leave you behind. Better to walk shorter each morning (when it is cool), enjoy the beaches in the afternoon if you wish, and form community with those pilgrims who also like short days, of whom there are sure to be some.
Just my two cents. I hope it helps.