Trade secrets are a thing, and enforceable in California.

> Would my current employer in this scenario be able to sue me?

They can always sue you even if it unlikely they would win.

> What if it’s “my own time” during the day, but I’m working from home and I switch from my company’s work to my own for 20 minutes before going back to my employers?

> What if the project is only tangentially related?

You should speak to a lawyer. It will depend on the details and your exact employment contract.

Short answer what does your employment contract say?

I live in the southern U.S. Non compete clause are 100% enforceable here, and have a ton of case law backing them. I generally won't take a job if it requires me signing it.

I have more than 20 years of experience, and I personally feel I am far more likely to bring knowledge to them rather than then providing it to me.

Anecdotally, regarding IP assignment policies: All large and medium-sized companies I've worked with in the Bay Area have a section in their employment agreement that says something to the effect of "We own everything you work on, using our equipment or yours, on our time or on yours, on company property or at your home."

California law restricts employers' ability to claim ownership of inventions made by employees on their own time.

(Self-cite: A flow diagram with statutory citations at https://www.oncontracts.com/docs/Who-owns-an-employee-invent... )

Its normally only "related" to your day job so in your example yes.

But if say your an amateur musician your day job employer has no lien over your compositions.