If I sum up correctly the memoirs of the time, the spanish war was more considered a struggle against guerilla with time to time regular operations against a weak spanish army and (at first)a small and distant british army.

This was the kind of operation which Napoleon delegated to his marshals : Naples from 1806 on, Silesia 1807, Italy 1805, 1809, 1812, Hanover 1805. And after Vendée, Calabria, Tyrol, he could be (over-)confident that the french army will ulitmately crush the spanish resistance.

Was it an underestimation ? Militarily, yes, Napoleon underestimated the difficulty of the task and the mutual support of guerilla and regular armies.

But strategically, no : Spain was not a decisive theater of war, due to its geographical position, feeble ressources and to the slow development of the situation there. Loss by attrition was important, but did not disable the units stationed in Spain.
And Napoleon had sufficient troops left to engage more strategical campaigns elsewhere : Austria 1809, Russia 1812, by putting new troops in a frame of experimented NCOs and officers.
Only in 1813-14 did the inconvenient of the two-fronts war appear, but who could have anticipate losses on the scale of the Russian campaign ? And as Digby pointed it, without war of Spain, the old units committed there would have disappear in Russia, leaving Napoleon in the same situation : to recruit a new, unexperimented army to face the coalition of Russia, Prussia and Austria.
So with or without Spain, the strategic decision was to search in Central Europe, not in Spain or Italia (by the way, the peninsular situation of Spain did not give great advantage to the british : good harbours were few, the roads to the interior difficult - see Oman, 1st volume. The same can be said of Italy).

So in the chain of events, Spain was important politically, militarily and strategically, but not decisive.