Well, Dianthus is great for many applications. You might also want to look into Woolly thyme (Thymus pseudolanuginosis) - it stands up to foot traffic, is quite nice looking, flowers nicely, and spreads to three feet, so you wouldn’t need massive amounts of plants (you can also easily divide the plants you buy once they are established). Woolly thyme looks fantastic around pavers and stones; you might want to consider a couple or three ground covers in your yard rather than the grass monoculture - some Dianthus further away from the walk, and Woolly thyme around the rocks or nearest to the pathway. You’d get the green ground-cover, and you’d also get fields of flowers (and Dianthus has a lovely clove scent).
I’d be really tempted to throw some Day Lilies and Asiatic lilies close to your house, too (maybe some Asiatic lilies close to the fence, since they are mostly vertical). The grass doesn’t seem to be thriving there (possibly because of the slope - drains the water away too fast). You could also move your Fountain grass there.
Your fountain grass does seem pretty big - you need something more like a Festuca glauca (Blue fescue) there, or a Molinia caerulea ‘Variegata’ (Moor grass), or Helictotrichon sempervirens (Blue oat grass). They all form smaller, tidy mounds, rather than a large, fountainy plant.
If you can find a lower water usage grass seed, what you can do is gradually replace your grass with it. Just seed your lawn with the lower water grass, then underwater after the new grass has established - the new grass should grow, and your old grass should die off. This is a process - I think it would take a couple of years to get a lower water usage lawn this way.
I really like the idea of using a variety of ground-covers, though. Multiple types of plants are always a good idea; nature abhors a monoculture. If you switch to groundcovers (and I am literally making it my business to help people get their yards off grass ), make sure the grass is good and gone first, though. It’ll act like a pesky weed if not.
Hope this helps!