Gorgeous house!

Oh, yes - beautiful porch/veranda! With all that filtered/part shade maybe a few of the Soft Caress Mahonias would work well as one of your foundation plants. Garden blogger Pam Penick at Digging has written about how much she loves it in shady areas. It's related to the spiny Oregon Grape Holly but has soft, palmy-looking leaves.
There are a lot of bulb-type flowers that will grow under pecans, like iris, School House lilies, Spanish Bluebells, etc. because the pecans leaf out late enough to let them get sun in winter and most of the spring. Downside of a bed underneath them is you're always cleaning something off the plants - leaves, dropped male flowers, pecan hulls, etc.
Have fun!

My great grandparents owned a very similar looking house in the historic King William district of San Antonio. I don't know if you're interested in a historically accurate landscape, but in case you are I just looked an old photo and see a banana tree in a pot on the porch and rose bushes by the steps. I'm going to dig out some photo albums later and see if I can spot any other recognizable plants. I know there were pecan trees because my father talked about them, and I seem to remember spotting aspidistra in another old photo. I think in general the landscaping was pretty simple letting the house itself shine as illustrated by Yardvaark above.

P.S. I googled King William District San Antonio and see that most of the restored old homes have very little in the way of foundation plantings. I'm assuming the yards reflect the styles of that day. Most likely there is an organization that advises if not mandates the landscapes.

This would be continuing my same scheme with plantings beyond those at the foundation. It looks like there is some slope at the street.

To hell with the no plants on the outside. Buy plants and lots of them. Ha! I could see some cast iron plants an in the shady areas . Plant delights has some white tipped ones that attract me . I could also see some lolly Jackson Mexican salvias. They love an amended clay And some filter shade/ and sun. Part of the reason for the lack of plantinfg was that often water was not as readily available and irrigation was harder. Maybe they were worried that indians were hiding in the bushes in the same way that police tell you that lots of planting make it easier for burglars.LOL.

Oakleaf hydrangeas will do alkaline clay if amended with compost. It would look killer with that house and the filtered conditions.

I envision a row of azaleas on the straight porch side and a bank of blue plumbago all the way around the curved corner. My blue plumbago at my last house (Temple) did well under the dappled shade of a much younger pecan tree. Here's a photo of it before I sold that house and moved 8 blocks away. [img]https://i.imgur.com/QlbnYNE.jpg[/img].

Another vote for oakleaf hydrangeas - they do quite well for me and I have alkaline clay soil too! I think the classic look of a hydrangea would be perfect with your historic house as well. :)

Maybe some purple lorapetalum would look nice contrasted against the white paint as well.