With that amount of time from landing to shuttle departure, you should be fine even if you are delayed a bit. Renting a car is another option like you said and no it’s not completely crazy to finish the drive after sunset since 606 is a well traveled route. Just take your time and use caution. In our opinion, a car would be nice to have once you are in Monteverde. Things are a bit spread out there and it’s nice to explore on your own without relying on taxis. If you do rent a car, make sure to save some money with our discount. More info here.
Answer for Liberia to Monteverde in the afternoon