A few comments:
1. I've also done 281 up through to Dallas - as well as US 83 from Brownsville to the Canadian Border.
2. As far as US 281 goes:
- Entering San Antonio from the southbound is OK, but exiting San Antonio northbound, I would skip US 281 the immediate area from airport north and use smaller roads. There has been perennial construction, closed/non-existent shoulders. Where it isn't under construction, parts are major freeway. A few miles north of 1604 intersection the highway settles into configuration it has until DFW area - a mix of one lane each way with two lanes each way. Generally OK shoulders in places with one lane - and two lane with light enough traffic to enable passing. It is still a major highway with higher speed traffic, but I found it reasonable to cycle. I haven't continued beyond Stephenville.

3. As far as US 83 goes:
- Here were trip reports - http://www.crazyguyonabike.com/doc/mev-route83-2018 and http://www.crazyguyonabike.com/doc/route83-2018-part2
- It goes further west than you had proposed.
- Still a major highway, but on average less traffic than US 281. Some areas in Kansas/South Dakota/North Dakota potential alternate routes close by.
- Existing "Fans of Route 83" Facebook group and some books on the route as well as friendly community