I had the same thought.

That 5.15% rate you saw was offerred by Bank of China.
A higher 6 month rate is offerred by Truist Bank a Canadian affiliated bank (TD).

I am willing to give up .05% in yield to support US based banks
vs. financially supporting China when a domestic alternative exists.

For liquidity, like you, I am considering locking in a slighly higher rate than MM SPRXX with these CDs through Fidelity.

Did invest a very small amount with NLY-PF recently.
If rates decline, preferreds may be a good alternative.