I'm Already Signed to a Label. Should I Still Use Songtrust?

The answer is most likely a resounding "yes!" that you should use Songtrust to collect your publishing royalties.
It is important to know that there are two types of ownership when it comes to music--the 'recording' / 'master' side, and the 'composition' / 'publishing' side. Traditionally this is broken up by labels (recording), and publishers (composition).
Songtrust acts as your publishing entity, collecting your publishing royalties for you. If you want to collect now, use Songtrust. It can take years to shop around for a more 'traditional' deal that works for you. In the mean time, make sure that you aren't losing out on royalties.
The only reason you wouldn't be able to use Songtrust would be if you entered a 'publishing agreement' with your label. In that case, reach out to them and see if they are using Songtrust to power their publishing!