Would like to post a few more pics but not sure how.
This is the slide area.

Is what I've outlined in red the area in question? If yes, for starters, I'd enlarge the island by moving its edge to where the yellow line is ... in a fluid arc. It doesn't look "at peace" when the tree trunks in an island are too close to its edge. If these are large shade trees, you'll be enlarging the island again some years down the road when the grass refuses to grow in the shade of the canopy. Keep the canopies well raised to minimize confining the view and producing dense shade.

I don't have suggestions for a 12" ht. grass you can seed. If you seed grass, there will be many weeds that come up along with it. Also, consider that when this is a shady place in the future, whatever sun-loving things you install now will not be happy. I would consider looking for a groundcover that tolerates some sun now and will be happy in shade later.

If I were going to plant plants here, I'd work on getting control of the weeds. You could kill them all and then mulch heavily with free tree trimmer mulch. Over time, you could install plants as fits your schedule and budget.

Yes, you've outlined the area in question. Those trees were planted to be shade trees in the future and I forgot to take that into consideration. Can you suggest a ground cover that would be suitable? We'd need a way to keep the groundcover from taking over the lawn (at the red/yellow line).

I was hoping for seed since the area is so large. But I guess with a good groundcover choice, we could work it in sections so it's not too overwhelming.

I agree the weeds are the main priority now. Not sure about mulching the area though. Its quite large and when we have mulched around new plantings, it quickly washed down the hill. The steepest part is at the red line which is why we stop mowing there. To the left of the stone path is an area where I killed the weeds and added some plants, pic below. I just don't know what to do with the bare dirt that's left.

I saw a website the advertises a seed mix of low growing grass mixed with clover. Says it won't get over 14" tall. Anyone have or heard success or failure with this seed or something similar?

You don't have a photo that indicates the slope so I can't swear to it, but I've had pretty good luck with the tree trim mulch staying in place (more or less) because the mulch is coarse. If you control weeds and then begin to establish a groundcover that is vining, it goes a long way toward establishing itself. Also, it's usually easy to create new plants by cuttings so it can be increased more easily. Frequently, people don't worry where groundcover bleeds into the lawn as mowing itself brings it to a halt before it can get very far. What are people there using for groundcover in similar situations?

I have a few suggestions. I am near DC, so I know a few things that work in this area although I think your temperature swings are larger.

First, I really love the wild look of the grass and the soft subtle shadings from gold to russet. It looks from a distance very lovely and a great transition from the lawn area to the woods.

However, I think you are asking for a controlled wildness with little to no upkeep and no mulch. My instinct is that if you spread the low growing grass mix over that area, the more established grasses would beat them out. It would be patchy at best and you would have to weedwack anyway.

Here are my suggestions. I think it would be lovely to plant that hill with daffodils -you would probably need hundreds - let them come up, then weedwack in early summer as the leaves of the daffodils die (don't cut beforehand or they will not store enough for next Spring). There is some type of attachment you can buy for a power drill/driver that will make the job super easy. The bulb forum is rather slow, but very knowledgeable. For summer to Fall, you would be stuck with what you have now.

An alternate suggestion is to use cardboard/paper to smoother what is there and then plant perrenials with high sun/low water tolerance. There are lots of lovely hardy sedums and day lillies come to mind. I really love lavendar also. Maybe some bushes for height/erosion control. For that space, it would be a LOT of cardboard! The reason I suggest cardboard is you can use long garden staples to keep it down. It would also become dirt and need to be renewed to continue suppressing weeds. Big project every few years and to make it nice looking, I would layer fall leaves or mulch on top. Look up lasagna gardening for more details.

Another choice is to buy a meadow mix, throw it out there and let the plants duke ii out --survival of the fittest!

Hope my ideas are helpful! I am not so much trying to give exact instructions as to point you towards possible paths where further research can help you reach the best conclusion.