We typically go south from DLR and we were not fans of Santa Monica beach/pier when we went north in 2019 (we did love the Warner Brothers tour and Universal etc). We find the beaches from Newport Beach down towards San Diego to be much nicer. We haven't been in March, but I will say in early June (June gloom), the weather is not always warm enough to be good beach weather. Nice to walk up and down in a long sleeved shirt and jacket or sweater, but sitting out in a swimsuit or swimming might not work. Someone else that has been in March can chime in on the weather that month. If you are looking for a beach close to DLR, Newport Beach is a nice area. There are also a lot of places between the
Disneyland area and San Diego on the coast that are nice.
Have never been to Joshua Tree- you might be looking about a 3 hour drive to get there.
The whole area around San Diego is really nice. Depending on where you go it can be anywhere from an hour and a half to a two hour drive from Disneyland. We have made that drive many times. For years, we would spend half the week in the Disneyland area and the other half in the Carlsbad to San Diego area because we had good friends that lived in that area. There are a lot of things to do in the San Diego area, and it is fun just to see that area too.