Here is a southern Cal idea for you, the San Diego Trans County trail. starts at the Salton sea, and ends at the Pacific Ocean. There is a facebook page with folks who are willing to help others with logistics, drop offs, etc. six days is likely not long enough to do the 150 miles, but then I don’t know your hiking style. The trail goes from desert, through mountains, skirts some urban areas.
Some of the logistics for the hike:
Resupply Logistics
000.0 Salton Sea- end of crystal ave in Salton City. Begin. (take 2 days food)
038.0 Borrego springs- grocery, outfitter, diner. (38, take 2 days food)
077.5 Sunrise hwy- possible resupply drop if you have a hookup. (39.5, take 1.5 days food)
083.8 Lake Cuyamaca- small store, restaurant. (6.3)
102.5 Ramona oaks Rd- liquor store, taco shop, pizza 4 mi west. (18.7, take 1 day food)
121.0 Lakeside- mini mart 1 mi south, then west on willow Rd. (18.5, take 1 day food)
133.6 Costco- food court, resupply, .7 mi south. (12.6)
134.7 Community Rd- In-N-Out burger .5 mi south. North 1 mi, Vons, Sprouts groceries and many restaurants. (1.1, take 1 day food)
138.8 Springbrook dr- 7-eleven, Phileas Foggs beer bar .25 mi north. (4.1)
149.6 Roselle st- mini mart/sandwich, taco shop, Arabic restaurant. (10.8)
153.0 Torrey Pines State Beach north parking lot. End. (3.4)
Water Logistics
000.0 carry water in.
013.6 water cache in wash- NOT supplied/maintained outside group hike. (13.6)
037.5 Borrego Springs, tap. (5.8)
044.3 Peña spring, .25 mi NW. (6.8)
051.6 Stuart spring, sulfur. Avoid if possible. (7.3)
053.2 Angelina spring, recently burned and ashy. (1.6)
057.5 possible water cache near hwy 78. (4.3)
069.0 possible water cache off hwy S2. (11.5)
083.8 Lake Cuyamaca tap. (14.8)
100.6 Cedar Creek, hopefully flowing. Boulder creek is more reliable 1 mi south. (16.8)
102.5 Tap at trailhead, restrooms. (1.9)
117.7 Tap in picnic area, restrooms. (15.2)
120.5 - 122.5 Yogi water in neighborhood if necessary. (4.8)
133.6 Water at Costco. (11.1)
138.8 Water in shopping center .25 mi north. (5.2)
142.3 Fountain at trailhead. (3.5)
145.5 Peñasquitos creek falls. Avoid drinking if possible. (3)
149.6 Water at mini mart. (4.1)
153.0 Done. (7.5)
Total estimated length- 153 miles.
I have camped and hiked Joshua tree, but I don’t have any speciific recommendations for backpacking there.