7-8 days to go from Santander to Belgium, via the Pyrenees is doable for sure.
All you have to to now is decide whether you spend more time in the Pyrenees and less time crossing across France to Belgium / Holland (and which way to cross) or visa-versa. As you are taking time out (30 hours) for the sea crossing from the UK to Spain, I guess the Pyrenees trumps France? But, there again, you might see the total journey (Holland > UK > Sea crossing > Spain > France > Belgium / Home) as one, all of equal measure, perhaps?
I wouldn’t necessarily write off putting in some motorway somewhere. I don’t subscribe to the “I never take a motorway anywhere” mantra but that’s just me. I’ll gladly spend a morning, an afternoon or, if necessary, an entire day on a motorway if it (a) cut out something that was not too special (the world is not always full of ‘great roads’) or (b) If it meant I could spend longer somewhere else, where there were indeed ‘great roads’ (c) It just meant I could do something else, somewhere else. Others might see this as some sort of heresy but, you know what, I don’t care.
For example, when I went from Millau, to Provence, to Chambery, to the Morvan, to Verdun, to Calais, I knew I wanted to spend two full days in Provence and one full day in Verdun. In other words, three full days when I wasn’t actually going forwards towards Calais. I was also camping, which (whether bods like it or not) does take more time than simply rocking up at a hotel, eating, sleeping and departing. I therefore inserted a chunk of motorway between Chambery, past Lyon and up to roughly Macon, in order to have a decent ride through the Morvan, to my next stop at Quarré-les-Tombes. The morning spent on the motorway, meant that I could have my two full days in Provence, my full day in Verdun and take D and N roads between Millau and Calais; to my mind, it was a good trade off. Of course, I could have sacrificed one or both of the days in Provence (or missed the area entirely) or skipped the day in Verdun….. or not gone to Chambery at all….. or done something entirely different between Millau and Calais….. or not gone to Millau….. or spent longer going along the Pyrenees from Pau to the Mediterranean, instead of the two days I took….. or not of had a full day off in Millau…… This shows how many potential variables there are between Pau and Calais, which is not hugely different from your intended Santander to Belgium.
Let us know what you rough out for your journey, please.
PS The way I went between Millau and Apt, in Provence, took me through the wilderness between Millau and Uzès before hitting Avignon in the local rush hour. Taking the D roads that I took between Millau and Uzès, is not quick. The roads are often narrow, sometimes not more than a car and a half wide, switching back and forth across valleys and gorges. It’s great fun (if you like that kind of thing) but there is nothing; no fuel, no lunch stop, it really is a wilderness. I knew this before I set off, with a full tank, a full Camelback and a sandwich. Had I just sailed out of the door with a quarter of a tank and no water and thought, “It’s only 100 miles”, I might well have been in for a shock. I should have jumped on the motorway to skip past congested Avignon at 33 degrees centigrade in the afternoon….. stupidly, I knew this but somehow forgot. Hey-ho, that’s life.