The S9 is known for its extremely bad battery life. It has a relatively small battery capacity compared to the phone’s powerful hardware. I personally have no experience with battery life on stock ROM and therefore can only tell you how it behaves with /e/.
There’s been an /e/ update in December (I think) which drastically improved battery life after my perception, so with normal use I usually get around 5 hours of screen-on-time now.
With GPS, the battery really drains fast and I experienced the same that the phone actually discharged while still being connected to a car charger. But I don’t know if this is specifically connected to /e/ or as mentioned before, to the powerful phone hardware. Maybe also try switching between the different GPS modes. “High accuracy” drains the battery faster than “Device only”. If you are not driving directly in a crowded city, then “Device only” would probably be enough for your needs.
However, I discovered that this obviously depends on the charger you use. Usually, USB ports in your car radio do not support any kind of quick charging and therefore limit the output USB current to 500 mA according to the USB specification. So you actually charge your phone with a power as little as only 2.5 Watts. That’s only a quarter of the power a standard wall charger would give you (around 10 Watts, without any quickcharging).
I would recommend you to buy a 12V USB charger which plugs in your car’s cigarette lighter. Try to look for one with quickcharging support like Qualcomm QC or similar which will allow you to charge your phone with 15 W +.