So I guess I need to eat my hat. Turns out the creaking from my cassette/hub was due to my hub needing to be re-greased. The odd thing was the creaking would only start after an hour or more into a ride. So I assumed it was the chain loosing its lubricant. I took apart the hub, cleaned it, and regreased, and now the creaking is gone.
That said, I did a bunch of reading on Squirt, and one of the known issues with Squirt (and Smoove) is that the higher viscosity means it takes a while to really penetrate the chain - and chain wear in the initial 10-20 hours can be very high as a result.
So I think I’ll go back to using finish line dry which has served me well for years.
For more than you might want to know about squirt and lubes in general, here’s a good read.