I've played many other smaller nations (especially after playing Byzantium) and achieved alot. But the main reason why I didn't list any of them is because the experience wasn't unique.Looking at your list of nations I think you're bored because you keep playing powerhouse nations (unless you played a opm in Italy like Lucca).
Try something harder like playing Provence or Nevers, forming the Netherlands starting as an opm, going for the 500 province Najd achievement, form the sultanate of Rum as Karaman, playing Norse raiders on the island of Sicily (that one is fun).
And you can always make things more challenging by playing on Very Hard as well!
I find that when things are too easy is when it gets boring.
With Manchu you have to time your attack period, corruption, loans and devastation when you try and be emperor of china
With the colonial nations you need to take advantage of Treaty of Tordesailes (whatever it's called), colonists, and strong relations so that you can colonise many parts that block other european powers.
With Russia you have balance your economy growth and military as you keep butting heads with the Turks and Poles.
Meanwhile playing minor nations results in a lesser experience as most have generic idea sets, almost no unique events or decisions to take, generic mission trees and restricted ideas to take because most national idea sets will almost never synergize with their standard idea sets. What results is (as I said) a lesser experience of more unique nations...