I am west of Austin in the hills above the Pedernales river. I would not do Japanese maple unless you want to gamble with your money. The shade is the only place to put them out here. Sandy loam means you have a lot of drainage so you will be pouring that water on it in the middle of the summer. No flame leaf sumac is not the same as a Japanese mapple. How about Bigtooth maple ( Acer granditentatum) but make sure that it comes from the lost maple or fort Hood maples. They are indigenous to Texas. They will end up a big tree. They like alkaline sandy loam and can put up with the drainage. They might loose their leaves in a harsh summer if watered with too much salty water. They will grow large and can do sun and partial sun. Chinese Pistache and even Texas pistache are good trees too, for color.Escarpement black cherry (but protectit while small from the deer). Redoaks are good for color but pick them out from the store when their leaves are turning because some just turn brown. Shumard Oak might work in your sandyloam if you have some depth to it (I am jealous of your soil). Smoke tree (Cotinus obovatus)might be a very nice substitute if you are looking for an understory collor and they bloom. These come in many varieties with differing colors, Like the variety Grace. The native variety will be tougher and remain small. If you haven't bought it, you need to get: Native Texas Plants; landscaping Region by Region by Sally Wasowski. Then make sure you deep water the tree twice a week not once a week during the summer because of the drainage factor. Usually they say once a week and maybe that will be alright for you. I am on the top of the hill on limestone rubble and I need to do it twice a week. I also needed to do it this summer to 6 year old trees and I didn't and they died. Oh, I was told another trick and that was to dig Square holes. The roots will hit the corner and be forced to pierce the hole wall. Odd theory but it makes sense. Then the roots don't end up going round and round.

Your area is certainly challenging for plants like Japanese Maples... maybe you could try some in large pots, retaining the ability to move them around according to sun exposure, etc and also be in full control of the soil characteristics... They tend to be quite small and best viewed close up anyway. They seem to do fine here in Northern NM as long as they're well protected (from cold, wind, and snow as well as bright sun). And by the way, you get double the precipitation that we get over here!

Never having gardened in your area, I'm not sure, but a few related tough native trees with nice fall color came to mind: Boxelder (Acer negundo -- TX native), Silver Maple (Acer sacharinum -- native farther east), Sweet gum (Liquidambar styraciflua -- TX native), Western Soapberry (Sapindus drummondii -- TX native). I doubt these are as commonly available at your local box stores, but when they are available I'm sure the price is much less than for Japanese Maple.