The fact you pointed out you already had this done with another camera, is precisely one of the reasons why I would strongly advise you go down the FTP route, since most (if not all) cameras will support that kind of functionality, so whenever the camera eventually needs changing, it is going to be a whole lot easier to change something as simple as checking the name of a file in an FTP server (which can be the computer doing the database logging and checking), than it is to change lower level camera-specific functionality, had the software been done this way the first go around, this time all you would have had to do was change one or two lines of code.
As for needing to check it every second or even multiple times a second, that is not a real constraint for not using a medium storage solution (FTP server, NAS, or microsd card), an FTP or NAS use either HDDs or SSDs, which can handle simple file name queries for breakfast, and microsd cards it is also a complete non issue, I would say it is a non issue for every single one of them, they are used as the OS storage mediums for computers like Raspberry Pis, but in this specific case if you end up using a microsd card (which is only needed if you aren't using an FTP service), you are going to want to invest in something like a WD Purple microsd card or one of its equivalents from another brand, since you want bombproof reliability on it.
So yeah, given the requirements you are mentioning now, and the fact you already had to change the camera once, I would say having the information saved to an FTP server which can be simply a service running on the computer doing the checking and the logging (assuming it is a computer and not a microcontroller), is not just a solution that works and is easy to implement, but it is actually the better solution IMO since you get ease of implementation, ease of future camera changes, and you get storage of images of the license plates and vehicles for future auditing in case anything goes wrong.
Going down the microsd card route instead of a ftp service is only advisable IMO if you have some very specific requirements where adding an FTP service would be less reliable, like say if you didn't already have a computer on site capable of running a simple FTP Service, which was my case (cameras are positioned on a battery-powered site and the only internet is 4G), I say this because the microsd card implementation is going to be more camera specific, and reliant on future cameras having similar ways of accessing them via web calls.
