Hi from Sequoia National park area! But I would not try to come here on that trip as I hate driving. Your nearest airport option to Sequoia is FAT. You could do Sequoia instead of PSP and then do a day and see Yosemite and fly home out of FAT if you'd rather tour the forests than the dessert. Gosh, I love the diversity of our geography, really anything you might want to see (fine, we don't have glaciers but we've got the rest of it) but it's spread out.
I'd price out the train to SD vs starting your car rental in Anaheim. It would be easier to start your car rental after your DL trip so you can go SD on your schedule and your partner will need the car anyway to explore SD. I would add on a couple days in SD if there is anything you want to do there. The Zoo is a full day experience and awesome (it's one of the best in the world)- but if you want to feed giraffes biscuits get there before 1pm! Can you tell who arrived at the giraffes at 1:05pm last Sunday?

Do the bus at the end of your touring time, short line and you'll want a break from climbing the hills. The Safari Park is also very cool, we did an amazing caravan tour there a couple years ago called the twilight caravan tour where they drive you into the animal areas and we got to feed giraffes leaves on the savanna and also a rhino! We did twilight because animals are more active then, the staff who drove us around were incredible knowledgeable. (The Safari Park is in Escondido which is about 45 mins from SD proper, plan accordingly.) Sorry I don't know much else about SD but I do know there's a lot to do- I just tend to be partial to animals

There's also a SeaWorld but it's been ages since I went so no idea what it's like these days.
PSP is incredibly easy to navigate if you can get a flight from there being a baby airport, SO much nicer than LAX in that regard. But you're not going to be enjoying any Starbucks at your gate and the bathrooms are outside instead of in the terminal area which I remember thinking was odd but like I said, baby airport so plan accordingly (there is a big section to the airport but it wasn't open when I was there, maybe it's only used for Coachella and other high traffic times?).