As a biking trip goes, Porto to Santiago is a pretty short trek. Even when I road the VDLP, what I carried with me was minimal. I rented a bike in Sevilla so I had a set of rear panniers, a good lock, a bike pump and a spare tube provided by the rental company. In addition, I brought my own bike tools plus a couple extra tubes knowing that they would most likely come in handy for flats and they did. Clothing wise, I needed far less than on the longer Caminos that I have walked. Biking clothes of course, a set of clothes for after getting off the bike for the day, rain gear, a few personal items, and my sleeping bag. All my gear fit easily into my panniers with my sleeping bag bungied on top of my rear rack. As you will be biking in May, you are correct about wanting to have sunscreen and of course sun glasses. But, I’d also bring along a rain jacket just in case. If I remember correctly, each pannier has a 20L capacity which is plenty of space for what you need to get to Santiago.
With all that said, a couple things where most helpful for me to have for a safe and successful Camino: 1) extra tubes for flats, 2) a handlebar bag, 3) a handlebar mount for my iPhone, 4) a portable charger, 5) a bike bell, and 6) flashing front and rear lights. I also lined my panniers with plastic bags in case of inclement weather which I did hit. Although both Portuguese routes are well signed, I like to ride or walk using the Mapy CZ app. With the battery charger in my front bag and plugged into my phone on the handlebar, navigation, particularly in cities, was quite easy. Bom Camino!