I had to replace my car charger because Google changed something preventing some Quick charge 3.0 chargers from charging Pixels rapidly. My Pixel 3 was like you said, barely just keeping battery level flat.
I just bought this one below and can confirm my phone now says 'charging rapidly' again and it increases charge even when screen is on. It's just as if plugged into a wall outlet.
I like this one because it's super descrete, low profile, and flush inside the car outlet.
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https://www.amazon.com/gp/aw/d/B07J...ag=hawk-future-20&ascsubtag=UUacUvbUpU6680031