The best thing you could do for that door is to figure how to add a roof over it to protect it from the sun.
Staining it a lighter color will go a long way to make the finish last longer.
For the best sealer money can buy look into this.
Far more UV protection then even Captains Varnish and about 7 times the protection of Min Wax Helmsman.
It drys fast, self levels, no sanding needed between coats in most cases.
The key in how long any sealer last is how many coats you take the time to apply. A rough rule of thumb is a year per layer between recoating.
Depending on the humity sometimes I can do three coats of Bristal in a day.