Semi-unrelated - for those of us with touring bikes that have a cigarette lighter in the fairing, the lighter assembly can be removed and replaced with one of these (or similar).
I had to take a dremel to hole the lighter mounts in to open the bore very slightly. It locks in solid and the same leads that run to the lighter will fit the usb socket.
I don't smoke and have no use for the lighter, so this was a much cleaner solution, for me, than using a car adapter.
Sounds like you have A/C, which puts the 12V at your shin. I have a stereo with USB but it's 1.2a which barely keeps my phone charged. I'm going to install a stand alone 2.1A USB panel. If you're hady with wiring, you can pretty much put a panel anywhere. You could even convert one of the vents into a panel by closing it off. Have a console? You can put it in there.
Have you considered charging the phones off the 12v side with something like this to reduce losses across the inverter?
I use these.
They're made to be direct wired, and they deliver 2.1 amps.
You cannot wire USB directly into a car. Amperage needs to be converted down.
Something like this will get you going for a hard wire solution. Make sure to use a fuse.
https://www.amazon.com/gp/product/B0082CXEI8/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1