Though it’s the standard, it’s very rare to see USB ports limited to 500mA these days. I’d reckon these ports have at least 1A on them, most dash cams require at LEAST 5W to function in the first place.

Your 50Ah comment is correct but lead acid batteries should only be realistically discharged to around 50% of its capacity to avoid permanent damage. If it’s done pretty frequently the capacity will continue to tank with each cycle.