And some more info here: https://www.happyrail.com/en/feve-narrow-gauge-network

It's honestly not really worth it unless you really like trains and visiting every railway line, or you happen to hike along the way and hop on for some stops (and, for the León area, happen to pass by at the same time as the daily train).

Critically: it only runs once a day! And Google Maps doesn't have reliable timetables there (as I also experienced last year).

This train looks like this, i.e. no amenities apart from a bathroom: https://cycling-on-rails.com/train-bicycle-guide/spain/#feve

The main advantage is that it makes 54 stops, so you can take it if you hike nearby.

From there you could:
- sleep overnight in the area,
- directly catch a TGV Hendaye-Paris if you're early enough and sleep in Paris,
- catch the night train to Paris from Bayonne (many changes so plan some buffer time),
- catch a regional train up to Bordeaux and sleep there.

#CrossBorderRail

While you are in San Sebastián, take your time and visit the center, it is one of the most enjoyable cities in Spain, I once lived there 3 years.

Going the Madrid - Barcelona - Paris - Brussels route is more reliable and faster because it uses high volume high speed lines both on the Spanish and French sides.

What I've read suggests that Hendaye is usually the faster route for north west Spain, and I'm not surprised planners are sending you via Madrid on the high speed lines. I did get routes further north with some effort and searching different sites, but AIUI Renfe doesn't show journeys with more than one change, and some journeys I found include Avlo.

Super useful train site