Can't help on the relays BUT I installed in 3 different cars extra USB outlets. You can wire them to an always on fuse using an "add-a-fuse". Blue Sea has many choices, Kussmaul, and some other amazon versions that get decent ratings. Some are made to replace a stock outlet. You could also just cut/splice a factory one if your decent with a DVM/test light and basic wiring skills. You can get the dual 2.4A, or a 2.4A and 1.0A.
I had my auto electrics guy install an alway-hot switched USB outlet inside the front seat console, and it works great. Just switch it off until needed. This will not link to the stereo, which is an advantage with other's iPhones activating CarPlay.