Iron mining (176 difficulty)
Picking agaves (152 difficulty - about the same time just from red skills)
[Gap]
Firefighter (228 difficulty)
Building a fort (224 difficulty)
Drilling for oil (294 difficulty)
The last three construction skills all have leadership as well. You'll get firefighter just as fast with leadership as con. You'll get building a fort faster with leadership, which is 2x, with con being 1x. Drilling for oil will take a little bit longer with leadership. Both are 1x, but there are 2 other red skills and only 1 yellow.
The only real reason to stay pure con is building, which is a very legitimate reason, but leadership is also used in building, so your building LP will still be going up, just not as fast.