I did 2 trips along the border in '17 and '18 and was intending to go back and do the northern section in '19, but that was cancelled as we moved to Portugal instead! A good move, although Covid has stopped us really exploring since then. Have a look on crazyguyonabike for "A Surly goes to Portugal" and "A Surly goes back to Portugal".
I did take some camping stuff the first year in case I was caught out, but didn't use it. Wild camping is not allowed in Portugal, but you can find campsite signs in most areas. Youth hostels, Pousada de Juventude, https://pousadasjuventude.pt/en
are good, reasonably priced and although I didn't base the route around them, found them to be in convenient places. Apart from one in Lisbon, they were pretty quiet. Hotels are also quite cheap, around 25 to 35Euros a night for country locations. You can pay more, but not necessary.
You also see hostel signs, although only(?) in tourist areas. The border region is pretty empty but wonderful for it. You can easily cycle for an hour or so without seeing a car. A few words of Portuguese will go a long way as English is very patchy, particularly among the older population who will be the only people on the streets during the day. But French is quite widely spoken and of course Spanish.
Feel free to ask if you want more infomation.
(I was hoping that 2020 would be the year of a long trip from home (North Wales) to Gibralter, but now, I'm looking at a trip similar to yours, north to Cherbourg or Caen, for a visit to Portsmouth, although don't know when. Of course, home to Gibralter is now just a weeks ride!)