Hi @Sammi, welcome to the forum 
Glad to hear it - sounds like a solid start
I’m a bit confused here - the rangefinder messages are sent to the autopilot, and get automatically forwarded to QGC. What modification(s) are you trying to make in ArduSub?
The rangefinder updates are at the same time as the velocity updates. If position holding is working fine I expect that must be faster than 1 Hz.
If I’m understanding your intent correctly, this thread may be worth having a read of.