travel Barcelona to SJPP - via Canfranc?

You would need to go to Oloron Ste Marie, part of this is on a bus operated by the SNCF.
Then changes at Pau, Bayonne, and Cambo les Bains.
Not an easy journey. Or its a lovely, if strenuous walk from Oloron on a fairly hilly GR.
Why not start at Oloron or indeed Canfranc? Arles route, which joins the Frances at Puente la Reina or thereabouts.

So there is a regional train from Zaragoza to Canfranc Estacion twice a day. At that point, the tracks end and you’d have to take a bus over the Somport Pass to Oloron and points beyond.

If you are not dedicated to walking from SJPP, I’d highly encourage you to walk from Oloron on the last stages of the Arles and then follow the Aragon to Puente la Reina where it joins the Frances. If that’s too much, start at Estacion and walk from there. I think it’s actually a better route than the CF.

I would agree it's a better route, the pass is a little higher, and apart from one road section a lovely walk. I suggest taking the bus past the fort unless you can do that section on a Sunday to avoid traffic.
There are actually quite a few good starting points along the way using the SNCF bus which replaces the train from Canfranc station to Oloron. It's well worth walking down from Somport to Canfranc. Which one you choose would depend on time available, the weather shouldn't be a problem in September but some accommodation might be closed. In 2019 most places that I stayed at had between six and twenty pilgrims each night in August.