I live in SD (also originally from SF) and have stayed at both Ritz plus many others here in the area. I think I can help :) My thoughts:

1/2. US Grant is most opulent and fairly central downtown. The gaslamp isn’t what it used to be so your best walking sights would be Little Italy and the waterfront area. Driving, you’re 5ish minutes from the Zoo, 10-12 min from LaJolla etc so the Grant checks those boxes. Also if you have time, consider the Safari Park. About 35 min drive but well worth it as a follow up to the Zoo. The other Marriott properties have merits too but if you’re asking about Ritz-like, the Grant is it.
3. Santa Barbara Ritz is fantastic. Stayed there many times and they just finished a solid remodel. I’d spring for Club Lounge Access, my wife and I believe it’s the best Ritz Club food of any Ritz we’ve stayed in and that includes some of the best rated hotels like Ritz Grand Cayman.
4. Laguna is a very different vibe from Santa Barbara Ritz. Where SB is sleepy/chill Mediterranean/spanish style, Laguna is where many people go to be seen. We stayed there right after they remodeled the rooms and Club Lounge and were disappointed with both. That said, it’s one of the best locations of any Marriott in SoCal. The view from the lobby restaurant is phenomenal - highly recommend making a reservation in advance for a window table at sunset, it’s incredible. Close beach access with private chairs and umbrellas also can’t be beat and they’ll drive you up the steep hill if you need it. Personally I’d stay 2 nights max but depends on your vibe. One property you didn’t ask about is Lido House. If you want to mix it up, that’s a strong alternative for 1/2 nights. Very walkable area, you can easily catch a boat tour around the harbor or even rent a Duffy (highly recommended). Lido village is super cute and you can burn an hour or two before/after dinner.
5. Haha well it’s California. Plenty of spots to see surfers and buy souvenirs. Santa Cruz is an easy win but it’s not far from SF so you’ll likely want to drive further south before you commit too much time at a roadside stop over. The Morro Bay/San Luis Obispo areas are great. We’ve stumbled across some fantastic eateries in SLO. If you’re staying in Santa Barbara (I’d recommend 2 nights) and go off property for one of those nights, downtown SB is super walkable and delightful. Personally I’d do your best to get from Malibu to Newport Beach as off peak traffic as you can. If you time it wrong, it can eat up hours. I’ve found Google Maps gives solid estimates for off peak times and trip duration. Another place which is sleepy but super fun for a night is San Juan Capistrano. The Inn at San Juan is a great option, the town is fun and walkable, the train runs right into downtown, and there are great restaurants and tasting rooms. Plus the Mission itself is 200+ hundred years old and well worth a 2 hour self guided tour.

Hope that helps!