I think 2 things should change when it comes to exploration.
1) land exploration should be twice as long. Maybe slow down the explorers or whatever, in my spain game i had all of the new world explored before 1520 except alaska.
2) expand how much is explored by ships. Aka, each mission discovers more land instead of targetting the one sea zone it uses now (or something like that.) I dont want a complete automation, since then it would be too fast.
Maybe make it so that you map out like half a colonial region's coast? half a trade company? or chain the missions together, like click "map la plata" and it will embark on several successive missions to do so on its own.