Hii Mike, welcome to the forum!
To answer your questions, the CXT595 is a low power, license-free FRS radio. FRS and GMRS share the same 22 simplex frequencies and channels, so yes, you will be able to receive transmissions from others that are within range on both services.
FRS can be used by both businesses and consumers, so you may pick up communications from surrounding businesses that use it. According to the FCC ID search (FCC ID BBO2155A), the CXT595 is only 1.3 watts on channels 1-7 and 15-22 and less than one half watt on channels 8-14, so you may be able to hear them but you may not be in range for them to hear you. FRS is allowed up to 2 watts, and those Cobra radios are well under that.
To clarify, the “security codes” you mentioned are a misnomer. Those are CTCSS tones and DCS codes that filter out transmissions from others that you don’t want to hear, but they can still hear you, if they don’t have them enabled on their radios. This short video explains what they are and how they work.
The antenna does make a significant difference, more so than wattage alone. Most GMRS radios today have removeable antennas that can be swapped and upgraded as needed, and there are a lot of great GMRS antennas available that are fine tuned to increase range considerably.
However, as mentioned earlier, Cobra CXT595 is an FRS radio, and the antenna is permanently attached because by law FRS radios are not allowed to have detachable antennas.
If you want to experiment, my advice would be to get a GMRS radio. It doesn’t have to be high end like the KG-935G Plus or something either. You can start off with a KG-805G and still get great performance because it is a full 5 watts, has a removeable antenna and can talk with GMRS repeaters to extend your range even further. You’ll find it’s a lot easier than trying to modify an FRS radio with an external antenna, which isn’t legal anyway.