Pet transport to STX

the temperature in the cargo bin under the seating area is the same temperature as the seating area...only the outer shell of the aircraft is insulated.
While on the ground, the cargo area is usually the ambient temperature of the airport tarmac....since the cargo doors are all open during loading.
The planes heating system circulates throughout the entire interior...while baggage stacks do slow the full flow of the air in that portion of the aircraft.
Please realize that it is usually 50 to 60 degrees below zero while in flight...so, the airline has to maintain heat throughout the aircraft....dogs and other living things fly very comfy in the cargo hold. On the tarmac it is a different story....that is why pets are first off and sent directly to baggage area.
My dogs have been frequent flyers between STX MIA and other cities.

See this link.

https://www.vimovingcenter.com/talk/read.php?4,275774

please check with your carrier to make sure your breed of dog can fly.

We used Amerijet for our 5 dogs and 3 cats in October of 2012.

They did fine.

Lay over was not a problem as they took the dogs out of their carriers for exercise and water.

As I recall they left Miami @ 11 PM or so and got here in STX about 1:30 PM.

All this after a 5 day across the USA car journey.

CD

Cocker spaniel and Poodles can fly, so I'd assume a combo of those would be able to fly as well. It's the snub nosed dogs that are not allowed in cargo.
We shipped our cars out of Jacksonville to STX 4 years ago in November, rented a car, drove to Miami with cat and Standard Poodle, spent the night near the airport. Booked the earliest flight out of MIA to STX the next morning. Trust me, we waited until the middle of November as the temp was right starting at that time, your flight in January will be ok. Much easier than the trauma of changing planes in PR. Great breed you have! Go for it. We gave our pets a dose of Imodium AD, just in case. No problems! But, be prepared for long, long check-in wait without Pre-check.

take the MIASTX non stop on American, as long as the temperature is not above 85 degrees in either location the dog can fly.

take the MIASTX non stop on American, as long as the temperature is not above 85 degrees in either location the dog can fly.

This....
I transported my 65lb 13 year old dog from California last month. He was fine. The journey from Miami to stx was part of his trip. I checked him in like luggage and paid $200
They fed and watered him.

I would be more worried about the stress at 11 years old but only you know your pets abilities.