Not that stilltrying is necessarily wrong but a lot can be done with shapekeys and its very dependent on your final goals. When I look at your current setup I don’t think you are aiming for the most complex all encompassing thing on earth. You want something that works well enough for basic expressions (not even speech) and shapekeys are fine for that.
The slider approach is a little old fashioned though. Usually multiple shapekeys are joined together in joystick like controls. The book is a little old by now but Jason Osipa’s “Stop staring, start animating” is a great resource for shapekey based facial rigs.
To get an inkling of how something like that would look there is this awesome thread on a web series called Boon & Pimento with very, and I mean very, extensive breakdowns of various parts of production including rigs.
I suggest you take a look, I think it will be very insightful. 
And in summary; I don’t feel you have to go of the deep end and go for full bone based facial rigs at this point. There are a lot of solutions in between that and what you currently have.