I will give some views of travelling in this area.
My personal views and experiences.
Take what you want and leave the rest.
I live in Algarve. So I have nothing to add when it comes o getting from UK to Spain.
But...
To fly to Portugal and rent a bike is popular.
Or to book an organized tour, that use this concept of renting bikes.
There are both tarmac and gravel versions.
It depends to large extent on the duration of your trip. One week renting is OK
For longer periods, it gets more expensive.
The time of year has a huge impact. August is the peak tourist period. July is also very popular.
That affects availability of accommodations and prices.
June and September are my favorites, If you want to include mountains. Pyrenees, Picos....
Else October is also very good.
The cost for accommodations drops drastically in early September.
Explore tourist spots?? Why on a motorcycle. If you want to visit tourist hot spots, take a charter tour. The MC is used to get out of the large cities and hot spots. I would not use a MC if I wanted to go to Lisboa or Barcelona.
Bike:
The roads in Portugal outside highways are mainly excellent tarmac roads, Twisty and with little traffic. Average travel speed is usually around 50-60 km/h. There is no use for a large bike. Himalayan or smaller is perfect.
(I had a Kawasaki GTR 1400 for some period. But it is now back in Sweden. It was not to much use in Portugal.)
You can combine the idea with renting and using a small bike:
https://europcarbikes.com/en/vehicle/honda-pcx
And avoid the cost of riding there....
And get a low cost for renting
Some examples for areas to travel: There are much more
Portugal
Douro Valley: Fantastic scenery. Best period is September, While it is still green.
North to South or reverse:
N2. Iconic road, You can ride it in 2 days, And see nothing. Or use 6 days to explore things around the road. It passes the Douro River. So it can easily be combined with some days in the Douro Valley. I did such a trip in October 2021. But the travel story is only written in Swedish. But links to the pictures
https://www.flickr.com/photos/191636466 ... 1688100586
https://www.flickr.com/photos/191636466 ... 1688100586
ACT: The proposed schedule is 5 days. That is rather intense riding. I would add a couple of days
Spain
Andalusia Mountains, Alhambra, Ronda...
North West Picos de Europa, other mountains, Santiago de Compostela, Burgos.....
Pyrenees Mix of Spain and France side.
Final comment
Just do it. This such a good biking area. You will have a great time, whatever way you do it.