I walked the
Camino Frances in May 2016 with my daughter and then did my first solo Camino on the Salvador (Leon to Oviedo) then Primitivo (Oviedo - SDC) in May 2017. It was probably one of the best experiences of my life. It was a very simple train ride from Madrid to Leon where I spent my first night and got rid of some of my jet lag.
The Salvador was exceptionally beautiful, beyond words actually. Others are correct in that it is very quiet with respect to seeing other pilgrims. I saw 5 in total during the 4 days I walked, however, I didn't feel lonely nor unsafe. I stayed in only one Albergue (in Pajares) because during that time of year you can be the only pilgrim in the Albergue. I preferred a cheaper 3 star hotel and there are several along the way. My recommendation would be to do the hike in 5 or 6 days rather than 4. Once in Oviedo, I began the Primitivo which was a quiet hilly version of the Frances. I would estimate around 30 people per day were starting the Primitivo and around 1/3 of them had been on the Norte and decided to visit Oviedo and join the Primitivo. There were ample places to stay if you don't mind mixing private and municipal accommodations. The only place I heard of people finding it hard to find an albergue was Grado (first night). Some had to move on the next town (5km) later. I would suggest doing the Camino Primitivo in 12 days (I did it in 10 and that was too fast).
Don't fret about the hills or being in the clouds. The people I met were all of average physical ability and aged in range from 20 to 70. They managed the hills quite fine. Regarding being in the clouds, there is only one place that occurs which is if you take the Hospitales option. I took the valley option to Pola De Allande as there is no accommodation along the Hospitales route and it would have required doing a very short day prior to going that way. The valley option was very beautiful and you still have to cross the mountain eventually. The walk up the mountain through the forest is very peaceful and beautiful.
When you reach Melide you do also join the masses of people and that can be a bit overwhelming but it is only for 2 days.
One benefit of doing the Camino San Salvador and Primitivo is that you get to experience 3 amazing small cities along the way (Leon, Oviedo and Lugo). Lugo is an ancient city with an intact 2.2 km long walkable wall surrounding the very active and pretty old town.
You should consider downloading the free guide from the CSJ to get more information about the Salvador and I also used the wise pilgrim app numerous times.
I am definitely going to do these 2 Caminos again, however, hopefully my wife can join me next time to share the beauty and meet as many nice people as I did along the way.