I found the bus was more convenient getting from SDC to Coimbra. If you take the SDC-Lisbon service, you don't change buses, but there is a break in Porto (at Campanha).
When I walked the CP in 2022, it took me six days from Coimbra to Porto. It is about 127 km, so that was just a bit over my target of 20 km/day. I could be walked in less.
I walked the Central route from Porto, so like you, I would probably look at another route. I have considered going through Braga, but won't have to firm that up for a while yet.
Swapping routes may not be all that easy. There are a couple of routes from Vila do Conde to Arcos.
Brierley's guide shows one, and I think Wise Pilgrim shows a slightly different route. Otherwise, I don't recall there being other convenient routes from the Costal/Litoral across the the Central. Going the other way seems to have a better range of options. Occasionally, someone posts a very useful map of the many options that are available.