I believe the rent-a-car system currently in place does not really provide the service it presents to new players. Before purchasing my own vehicle, most of the time I spent running to my destination as I was not near a car rental service at all. It's frustrating having to wonder through the city avoiding the chaos of courier drivers trying to get to my intended location.
Furthermore, there also needs to be additional areas where you can spawn your personal vehicle as currently, even with my own vehicle, I continue to run to most of my destinations. Maybe you should be able to spawn your personal vehicle using a command as sometimes people crash or have to leave the server in locations nowhere near a vehicle spawning service.
To counter the problem with car persistence, personal vehicles should automatically be removed from the world after 30 minutes of inactivity and rental cars should be removed after 10 minutes of inactivity. Hopefully this helps reduce the server lag.