I think that your best choices are the Invierno or the Salvador/Primitivo.
Comparisons —
Invierno has many fewer walkers, very little mountain terrain, basically a walk through small towns in rural Galicia. A fair amount of road walking, but on roads where cars are virtually non-existent. There are a few bigger places, like Quiroga and Monforte, both of which are nice places but not monumental like Lugo or Oviedo on the Primitivo. The day you cross the Miño is awesome — vineyards on both sides and gorgeous views going up and down.
Salvador/Primitivo has many more mountain days and more varied terrain. Probably less asphalt than the Invierno, though it has some of course. Oviedo is one of my favorite cities in Spain, I would take a rest day there. The Salvador has the most important pre-romanesque churches in Spain, so if you are interested in ancient architecture, a trip to the Naranco sites (a couple of kms from the center of Oviedo), and the church in Santa Cristina de Lena, are must-visits! Many more pilgrims on this route. When I walked two years ago in September, there were plenty of people, and some slight stress for lodging (particularly in Berducedo, which is a bottleneck). Because of covid (this was 2021), I booked private rooms ahead of time and had no issues. If you walk this route, I would make sure to have a reservation in either Berducedo or La Mesa (about 5 kms further on). Lots of good albergues on this route and plenty of private places too.
I wouldn’t recommend the Olvidado to @Payfac, but not for those reasons. The Olvidado is probaby the most consistently beautiful camino I have walked. Of the approximately 18 days, at least 8 are true mountain stages. It is a “new” camino only in the sense that it has had a recent spotlight put on it by the Camino associations, the municipalities, etc. But this route is every bit as old as the others. I first walked it in 2013 or 14 and there were some arrows but GPS was (and continues to be) needed. I went back in 2019 because two mountain alternatives had been added and marked. More albergues, more markings. It is the perfect camino if you can walk longer stages and enjoy solitary caminos. And I realize that’s not everyone’s cup of tea.
The reason I would not put the Olvidado on @Payfac’s list for this year is because by joining it in La Robla (after a day on the Salvador from León), he will miss much of the beauty. Starting in Bilbao (though the first day is all asphalt) would be my preferred starting point.
The one caveat is that this camino has stages in the 30-35 range. There are ways to shorten them, and there are cab options for some of the longer ones if you are not opposed to taking a cab now and then. I wouldn’t want to do that, but that’s just me.
Good luck with this decision
@Pafayac and let us know what you decide!
And a slight tangent to say that I vry much look forward to your posts,
@BobY333. Buen camino!