Is it a bad idea to switch between synthetic motor oil to standard motor oil and back to synthetic?
No. The viscosity is more important. If the book says 5W20, use 5W20, for example.
You can go back and forth and mix all you want. It’s perfectly fine.
There was (what I suspect was an urban myth, but will let the more knowledgeable people comment) the belief that synthetic would cause seals to swell and returning to conventional the seals would shrink and cause oil leaks. May have been started by makers of full synthetic oils, I doubt it is true.
Your 2012 requires Dexos-1 specification motor oil, so you will have to use an Dexos licensed semi-synthetic or full synthetic motor oil. I don’t believe any conventional oils meet the spec.
Let the owners manual rule: if there’s no non-synth that meets the spec, stay away from any non-synth. It’s a more recent phenomenon, that more and more cars have a spec that is only met by synth or a blend, but believe what the carmaker says.
There is no standard motor oil that meets the requirements for your engine. Any oil you use must have the “dexos” label on it, and that means a synthetic blend or full synthetic that meets the dexos specification.
If you can wait for Costco’s next price reduction on Mobil-1, you will be able to snag a Dexos-certified oil that is substantially cheaper than anyone else’s price.