It's not possible to book the municipal albergues, you are right here. Sepetember and begining of Oct is still a high season on the Portuguese, at least on the ways from Porto, Coastal or Central. There is more infrastructure available on the Central though, at least close to the end of the season. If you walk from Lisbon though you might be all right withot booking: there is enough infrastructure there now and no crowds of pilgrims. If you want the off the beaten Caminos where you don't need to book anything in the mid of summer then head to Portgues Interior from Viseu: we haven't met a single pilgrim for 9 days till we reached Verin in Spain, I posted about it here
https://www.caminodesantiago.me/com...port-from-portugues-interior-june-2018.56758/ Or to Camino da Geira e dos Arrieiros from Braga.
All the Portugues routes from Porto are popular throghout the year, we walked the Coastal mid Jan and it didn't look totally deserted either.
Bom Camino!