This route goes in the opposite direction: San Diego to Phoenix. The distinction is relevant because the stretch near Jacumba involves ~3000' of elevation change on the freeway shoulder. San Diego to Phoenix, you're going downhill. Phoenix to San Diego, it's uphill.

There is a limited number of passes leading into the coastal ranges from the desert. The most cyclist-friendly route is IMHO the one from Borrego Springs to Ranchita. As an added benefit, you can take S22 into Borrego Springs (light traffic), and this route puts you much closer to Los Angeles.

From Ranchita you can take 76 or 79, 79 is more direct for someone headed to LA, but there's traffic and there's no shoulder in some places. 76 is quieter.

P.S. The downside is that you have to make a fairly big detour. Wickenburg to Parker to 29 Palms is shortest. Wickenburg to Blythe to Brawley to Ranchita adds ~40 miles and puts you on I-10 for ~30 miles (but I think it's more scenic).

I wonder if there is a route along I-8 in Arizona. Google Maps show a road that goes from Phoenix to Yuma between I-8 and I-10, along the train tracks, but it appears unpaved part of the way.