Engineering Goby is a good callout.I would just be very careful with adding new fish because sharks are notoriously difficult to medicate and often respond poorly to most available medications. Personally, I think a larger puffer (dogface/porcupine?) would look fantastic. Some triggers may nip on the shark so be careful with those, a blue throat would probably be the safest if you wanted to try that.
If you really want eels with the shark, maybe try an engineer goby? Then your sand bed would be clean and you have an eel-ish fish that should be able to easily avoid the shark.
I honestly have no interest in triggers or puffers.
At this point, it's mostly just CUC. I was thinking the Dragon Goby as they can grow up to 6" and are extremely fast.