Since your plan is to do pasturing you need to research planned pasturing in order to maintain if not restore grass land.
National Geographic covers the subject a little: NatGeo
. and more on the subject can be teased out of The Google
You might want to research Swales on Contour
As once you start removing trees you will be opening up the land for water erosion.
Livestock do not need a lot of acres that are devoid of trees. In most cases one can have a 'bright shade' forest, that is a forest that is more spread out with a light and airy canopy and still have more than enough grass/fodder being grown.
I definitely would thin out the forest instead of clear cut, starting off with the diseased and dying pine/fir first. Definitely leave a line of firs on contour, meaning going side to side along the elevation lines to hold and maintain the soils as stable. For areas you wish to plant other trees, I would leave about 10' of space between the drip line of the Douglas firs and plant a hardwood or semi hardwood in the middle. The drip line is the edge of a tree's canopy. So if the canopy extends 8' from each Douglas fir, you are clearing a space that is 16 feet from trunk to trunk.
The issue with firs and pines are not so much that the trees are noxious and out to murder all other trees, its the needles that have high amounts of acids and other growth inhibiting chemicals that prevents other plants from growing/sprouting. So you just need to keep the fall of litter to a minimum and only rake out an are of about 24 inches away from your new planted trees.
Since it is Douglas Fir you have a type of wood that most dimensional lumber is made from. I would see what lumber agencies are in the local area and see how they would deal with getting to the trees and taking them out. If the property is large enough they might have to build a lumber road which means they will bring in bulldozer(s) to cut in levelish road, thus giving you a starting place for you to dig into the side of your hill and move earth to the down slope.
I think most lumber agencies prefer to clear cut, but you never know they may want to work with you depending on how much money they will get from the harvest.
Hills and mountains are usually hills and mountains because they are composed of sturdy material - rock. As such you would need to know what kind of rock you have. Is it a sheet form of rock like limestone? then most likely you can use excavated limestone to build a retaining wall on the down slope and have a sheet of limestone to build on. If its broken granite then the issue is digging out boulders or even having to crack them open some how in order to move them... solid granite might meet my favorite fishing tool trinitrotoluene
more commonly called 'boom'. ;)
The 15 degree slope isn't that bad. You are looking at around 32 inches of elevation being removed for ten feet of width/length. To put it another way, if you where building a house that was 20 feet wide with the floor level uphill, then the down hill side would need about 5'6" of wall to support the upper level. a bob cat could probably do the rough digging and moving in a day or two, depending on how large an area you wish to clear. If that lumber company are willing to play ball and if you are willing to plop your buildings in a more centralized area they might actually do the rough cutting for you to get at the trees.
Scrape down to bedrock, which might be readily available since you have springs at the top of the hill, which means water is being pushed up and out, not out the sides of the hill. If you don't find suitable bedrock then piers are constructed which are basically deep holes filled with concrete and rebar which act like bedrock.
As for planting trees. If you are buying lots of them then go with "seedlings", yes there will be a lot of loss that first year, but you get more bang for your buck and its far easier to use a post hole digger (hand operated one) to dig a tiny hole which you can plop a tree in, pack it below grade then spread a bit of compost around the tree with a mulch material. For deer and other plant predators tiny cages to keep the critters out is much more affordable, especially if you make them yourself out of chicken wire. I make 12-20 inch high cones that are 6-10 inches in diameter at the widest end.
When I did volunteer planting, we had a simple contraption for digging a seedling hole, called a Tree Planting Spade. One step, pul it out, plop in your seedling, dump the earth, tamp twice, move to the next planting spot, repeat.
Most trees are forgiving when young, as long as you protect them from predation (deer, rabbit, etc.) keep them moist and provide them a first year covering of loose mulch (because in forests trees drop leaf litter to protect their brood) most of the trees will survive.
If you are planting orchard trees, fruit trees then yes digging a massive hole and doing all sorts of labor to insure the tree survives the shock and thrives is needed.