I want to get a Garmin Astro 320 (or 430) which transmits GPS coordinates to a Garmin handheld unit on frequency 151.820-ish and 154.600-ish. But I would like to have the Garmin Astro GPS collar on my dog and receive the coordinates on a handheld APRS ham radio (thinking of getting the Kenwood D74A). Is this possible?
I don't know what protocol the Garmin 320 and 430 dog tracking collars use, but it is highly unlikely that they use AX.25 packet as their protocol, because that protocol is old and slow, and therefore harder on batteries. If they don't use AX.25-based APRS packets, then you wouldn't be able to receive the packets on a Kenwood TH-D74A.
No. These collars do not transmit in the ham bands nor do they use APRS.
While many ham receivers can receive outside the ham bands, the protocol used by the Astro is proprietary, best I can tell. So to demodulate and decode it with anything except Garmin equipment will require some reverse engineering.