The 3/8" Spectra should hold up fine for deep anchoring. It's tensile strength is about the same as 5/16" G4 chain.
Xsbank: The strength that you quoted for 5/16" G43 chain is its safe working load. Its tensile strength (ultimate breaking force) is four times higher.
See
https://www.westmarine.com/buy/west-...IaAj5dEALw_wcB
David