If you aim >= 30cm, you definitely need a 128kHz RFID system.
But I doubt that you can read >= 50 tags with RFID at all, because with RFID, each passive tag needs to be powered from the reader.
You might want to try BLE (bluetooth) technology. The latest versions (5.2?) of that standard supports angle detection of the devices.
http://www.circuitnet.com/experts/39998.html juki uses an array of antennas (presumably one per slot) perhaps wired in a matrix like this.
the antennas could perhaops be drum inductors (they's haver to be something with a small field so as to be sufficiently selective to only see one tag)
simulate this circuit – Schematic created using CircuitLab
This is a surface-mount RF module, so you need to make a PCB to solder it on, unless you do some freestyle wire soldering. You also need an antenna, a GPS module, a microcontroller, voltage regulator and battery.
For the device to run from a reasonable battery for a month, will take a strategy on when to power on the RF module and the GPS, and even to take ...
Of course, all emitters fall under the rules of the local regulatory body (which seems to be FCC in your case), unless there's a specific norm that exempts them (which seems to be the case in the US for tags, if you're reading that policy right).
ETSI is not a regulatory body, but a standardization organization, so they have nothing to do with this; the ...