Hey, I'm a huge Black Jewels fan, first off.
What you are doing makes sense. Sponsored Magic is not limited in the way that Ritual and Thaumaturgy are. Instead of picking areas of magic, you have power related to your sponsor's "theme."
Also, there isn't a distinction between Evocation and Thaumaturgy for the purpose of effects. You can use Thaumaturgical effects with the speed and trappings of Evocation (see the sidebar in the Sponsored Magic section). The trick is that a lot of Thaumaturgical effects are still really complex, and thus will still need a ritual, since doing them as evocation could quite literally blow out your mind (think of it as the sort of thing that could shatter your jewels when done in a rush, however when done slowly, greater power can be controlled).
One last thing: consider checking out the item of power rules, and using the jewel as one that grants you access to sponsored magic. Make sure you read up on the debt system under the sponsored magic rules and work out with your DM what would be appropriate, since they are going to be in charge of that. Running hot on debt and losing access to your powers could be the equivalent of draining the jewel, and needing to let it recharge.