For any planning tips on a combo trip, you check out an existing recent-ish thread: DLR and San Diego - first time
As for when to go in March-May- early, early March will be slightly less crowded until Spring Break crowds start which will go until Easter. I don’t mind spring break crowds as they tend to be dispersed over time and April-May has my favorite weather and the parks in the spring are just beautiful. Food & Wine will also usually be going on for some or all of this & that bring a lot of crowds too, especially locals at night but they also do a lot of great live band entertainment and if your’e at all a foodie, the booths (especially with a Sip & Savor pass) are a great way to try lots of new & interesting things (but definitely at a higher price point so it’s always easier to enjoy when you try to forgot that the 3 bites you’re eating were $9).
We’ve also gone several times for Memorial Day weekend and it’s never been too crazy and they usually kick-off the start of the summer with increased nightly entertainment (which is our favorite!) and so a great time for us to go.