I have walked the CP Central Route (from Tomar) in 2022, the Variante Espiritual (from Valenca) in earlier this year, and the CI almost a decade ago. They were all wonderful walks, and I didn't find any logistical difficulties with any of them. If I were going to do any again, I would probably choose to walk the CP from Vigo just because I haven't walked from there.
It is also relatively easy to get to both Valenca and Vigo by bus and Vigo by train, although the train does require getting to Campanha, the main Porto bus/train/metro interchange station. This is easy enough on the Porto metro from the airport, but might not be the sort of thing you want to face coming off a longer flight if you are not a regular traveller.
I know it might still be early days in your planning, but if you plan to return from Santiago to Porto airport by bus, consider getting your tickets a couple of days before you want to leave Santiago. I found that booking the night before, all the cheap seats that had been available on ALSA the previous night had gone, and the price increased substantially. The bus filled up at Santiago, with only a couple of people getting off along the way before Porto Airport.