Oh god. You're right Spain in 1519 leads a PU with Austria, is everywhere in Europe and has colonies and room to conquer almost all the natives. The world really is yours to lose.
Have you taken a look at Charles I von Habsburg, while we are at it?
Edit: now that I've give given it a try, 1810 France is so ridiculously powerful that even if you do go ahead and invade Russia, you can still emerge over all of Europe before 1820. Give one military stack Napoleon as a general, then sent it through southern Spain, instantly disbanding every rebel in the way, and then to Portugal, conquering it effortlesly. Simultaneously, Austria should have declared war on you; send two stacks with good generals and destroy Austria's two large stacks, it is easily done and your vassals will handle it from there onwards. Portugal won't surrender easily, but Austria does; you can force it to liberate Hungary, after which it becomes an useless cripple. Spain, once its mainland has been pacified by Napoleon, will successfuly conquer Brazil and if you start building a navy right from the start you should be able to hold the british navy in a battle for long enough to a few dozen of transport ships to get napoleons four dozen men stack into the british mainland, after which it becomes a curbstomp. Assuming you didn't waste time, as rebelleions would start popping up all over due to WE otherwise, you will have managed to take all of Portugal's european possessions, splinter Austria in a shadow of its former self and gain a permanent hold over a third of your choice of the british islands. You should have from 3 to 4 unused stacks, which can be sent against Russia; while the attrition does hurt, you can use the large stacks to defeat Russia's pre-generated military, then split them up in several small units which can be used to cover all of the russian soil, preventing further unit production. It may be worth sieging a russian province or two to use as supply bases while advancing through the territory.