The best way to get from Barcelona to Andorra and for traveling around Andorra is to rent a car. That will assure a necessary level of mobility and will allow avoiding the problems related to waiting for a taxi. One may rent a car either in Spain or France (at the airport or at a rental agency) or in Andorra.
In Andorra car rental companies offer a wide range of vehicles and prices. Some companies ask for a 100 euro deposit from a debit card of the client, which is returned when the car is returned. A credit card payment assures a non-deposit form of payment. At the airports of Barcelona and Toulouse parking for rental cars can be found at all terminals, including international ones. Thus, a car may be rented for the entire period of a stay in Andorra and then returned near the check-in desk of your flight. Look here for car rent contacts and prices: https://all-andorra.com/rent-a-car/