Your land and surrounds are interesting. The oaks appear to be about the same age which indicates that it may have been farmland until about 30 years ago. The land is disturbed and eroding. Your first expenditure should be hiring a landscape firm to hydroseed (without adding grass seed to the mix) all the loose soil. Then stay off the area for a year giving it time to stabilize. Frankly, I would not plant the slope. Let it revert to nature, weeds, brambles and the natural vegetation. Then concentrate on planting shrubs around the upper edge fading off into the trees. Do a search for "shrubs for shade + Missouri" where you will find numerous varieties suitable for your situation scattering them out and about. If you wish to spend the money construct a retaining wall in the sunny section along the drive, backfill and develop a perennial/shrub garden. Much easier to care for. This will take the emphasis off the hole in the ground.

I've come to understand that organic mulch -- shredded wood chips etc. has more than one function ... I inherited beds mulched with gravel. It's gone in one main area. It's "buried" in others. I've let it be in my iris bed because they like the heat reflection and drainage it -- the gravel -- has given to the basically clay soil of my yard. I wouldn't have done it that way, and the gravel, although still sort of looking like a "mulch" permits plenty of weeds to do their thing. Rocks, gravel, whatever have their own maintenance issues, not least of which is that weeds soon take up residence. Laying landscape cloth ... ask just about anyone here who has gardened for 10 years or more ... won't prevent weeds for long and becomes an immediate hassle when trying to plant something into the area. It doesn't take long for enough soil to be added on top of the gravel layer to make it possible for weeds to grow. And, trust me on this, weeding out of a gravel mulch bed v an organic one takes longer and is far rougher on the hands that weed the cradle ... I mean the flower bed. When you mulch by renewing an organic mulch ... 1. You are freshening your weed suppressant layer. 2. You are reducing your watering chore in a way that more sun reflectant, heat absorbent materials, like gravel, won't do. The organic mulches provide shade and moisture protection for plants. 3. You are actually improving the tilthe or structure of your soil. If you aren't growing any plants in your gravel that you like, then you can, of course, use weed killers. And there are some instances when gravel makes sense. Maybe this is such a case. In fact, I hear ya' on getting beds mulched each year. It's just that it wouldn't be true to give the impression to others reading here that gravel is a magical eternal weed barrier. it isn't. And, it has it's own negatives. Not least of which is the artificial or industrial look of it. Wellspring

I should have mentioned, in order to avoid distortion, post the pictures individually instead of making them into computer generated panoramas. Much of the front of the house cannot be ameliorated by plantings, since more than 50% of it is covered by paving. White may not be the best color for the garage door. Maybe it should be different than trim or body color. Seems like the left side could use a tree in an island bed and the tree @ right could use a bed. Some groundcover, perennials and annuals could screen the foundation. There could be a spreading shrub off the corner of the front porch. I don't think that retaining walls and terraces are necessary or that they would resolve perceived shortcomings of the overall scene. Part of the solution will come when the existing young trees grow to larger size.

It is impossible to keep critters from passing through a planted bed, but so long as you don't cultivate plants that pile up thickly with plant debris, they would not be inclined to make it a home. I hope it is just camera tricks, but the walk at the back of house appears to be sloping toward the house. If true, that would be worth fixing, as surely, a large amount of water will come off the hill at some point. At the end of the flumes, it appear that gravel is piled above the flume outfall height. This is going to create somewhat of a dam. It will drain out at the end of a storm but will hold some height of water during a storm when there is heavy flow. I would plan to be out there with an umbrella observing exactly what's happening in order to know if something needs attention. Do you need to be careful of wildfires? If so, see if the local municipality has a list of recommended groundcovers. Do you need shade at that side of the house? Some tree forms along the hillside would help hold the soil in place while not blocking the view. The steps to the top (nice wide ones) sound like a great addition if there can be a special viewing spot created there. What flanks your property on the other side of the hill? As one example, a tall bamboo forest (such as Oldhamii) with groundcover below, would look great. It looks like you have enough room at the end of the house for a more or less linear patio. One with some boulders could look nice.