Cofounder considering leaving after Series B - looking for advice *I Will not promote*

I'm a technical cofounder of a venture-backed startup that has raised several rounds of funding, grown to 70+ employees, and is now well beyond the early startup stage. I'm no longer on the board following our Series B and am considering moving on for multiple reasons. My first question is around the financial/legal side of leaving. I have a mix of equity types and want to fully understand my rights around exercising options, buyback provisions, etc. For those who have been through this, did you work with a lawyer, accountant, or both? What type of law firm specializes in this, and does anyone have recommendations for firms in California they've personally used? My second question is about timing and communication. While I'm financially stable, I'd prefer not to be unemployed for an extended period. Is it reasonable to secure another position first and then give ~four weeks of notice, or should I be having a conversation with my cofounders and board now about a multi-month transition? I'm still involved in several critical company functions, but we've grown enough that the company can continue operating without me. I'd really appreciate hearing how others have navigated this, particularly those who left companies they helped found.