Have done SdC to Santander using Renfe and FEVE but as
@Tincatinker says it will be slow but very pleasant. I had to cycle the Ingles to Betanzos to get the MD/Regional to Ferrol. You hang your bike on the rack. From Ferrol, it's wheel your bike on to the bike space on the train for Oviedo.
At Oviedo** you are back onto the MD/Regional to Santander. Took 3 days. Overnight at Ferrol and Oviedo as I wanted to see these towns - and wanted to cycle part of the Ingles.
The fastest way is Alsa Bus. Alsa sell
bike bags * so it can be partially disassembled and put in with the luggage. Pre-book the space on their website.
* Or buy the bike bag from Alsa (€15) and use it to go by train.
** At Oviedo station, the FEVE ticket office and trains at upstairs at the back of the station.