Sole proprietorship vs S corp for 1099 side gig in CA?

Hello I'm based in California. W2 career earns $350k with all the usual benefits at large institution. I'm starting 1099 side gig doing per diem with expected annual pay between $90k to $120k (depending on shifts). Based on this 1099 annual income amount, I'm told it's pointless to set up S corp by multiple people.

• should I set up an EIN?
• if I set up an EIN, will I also need to set up SEIN (California resident)?
• what accounts do I need to setup for quarterly taxes? Does this apply to both federal and state taxes?
• can I use the 1099 income freely to spend (or invest) like my W2 income?
• is the separation of my 1099 income from W2 income purely for tax purposes? Or can I freely transfer $$ between accounts?
• my accountant files my taxes throughout my W2 career. Will this person manage the quarterly taxes? Or do I do this on my own?

TIA.