From the sound of what you're doing with yours, have you considered a nice set of colored pencils. Your could do your drawing and shading with them, and then, of course some coloring. You can always blend them with odorless mineral spirits OMS, such as Gamsol.
If you did watercoloring, I would recommend Tombow markers, similar to what you've been used to with your SU markers.
Copic markers are great, but more expensive, and you mentioned you only use them occasionally, so I wouldn't go that route since that won't get much use. The other two options you can probably find on sale or at a more affordable price point in smaller color groupings.
Hopefully some others will have some good ideas for you as well, and good luck with your decision.