I just finished the walk, Lisbon to SdC in November. We also skipped the part out of Porto, on the recommendation of locals there who had done the walk before. We took a train, which was quite easy and cheap (maybe 4 euros?). We looked on our phone for the stop we wanted that was right along the Camino trail. I don't know the name but will ask my friend, as he handled this piece, not me. I think taking the train would be cheaper than a taxi and easier than a bus.
Regarding your suggested route: I said, as soon as we walked on past Porto, that a loop would be better than what we did, Lisbon to SdC. I imagined starting in Porto, going to SdC via the inland route, then looping back to Porto taking the Costal route. I personally wouldn't want to loop back to Porto from Redonella by bus to then do the coastal route. That would feel, for me, like interrupting my flow. But that's just me. I can see how (as I suggested) heading off from SdC to head down to Porto might not seem really great either, because a finish in Porto is somewhat anti-climactic compared to a finish in SdC. That's going to be a personal feeling that's different for everyone.
Regarding Lisbon to Porto, I'd say you're not missing much. Here's my summary notes on that stretch:
- Very long days
- No pilgrim office in Lisbon meant there was very little support for the journey. The Cathedral, which we stopped into twice, was not helpful in the least. They sold the maps (not full guidebook) and could offer no advice/ wisdom for the journey.
- The trip from Lisbon to Porto isn't particularly beautiful. There's some nice cities (Tomar, Coimbra, Ponte de Lima, & Valenca), but the stretches from the towns are often on paved roads and not always on green nature paths. Plenty of Eucalyptus forests though. If you're deeply effected by your physical surroundings and need lots of stunning nature, the CP from Lisbon to Porto would not be for you. If you can focus on your inner journey and not let your surroundings change your mood, then you won't have an issue with the route.
- We saw almost no pilgrims Lisbon- Porto. Maybe 3 in a day, but not every day.
My full note on this Camino route can be found here:
https://www.caminodesantiago.me/community/threads/lisbon-to-santiago-october.64544/#post-801606
Hope that's helpful! Bom caminho!