I stay at various Least Eastern properties pretty frequently, so I'm a rewards member with them. I can book probably up to January with them directly and there will be vacancies.
This year, though, my wife and I are booking a four room AirBNB and bringing her parents to watch over our kids. We're making it a full family vacation, and the grandparents get to spend time with their babies. Total cost is $120/night - not per person, but total.
There are also tons of hotels that don't book/use the TMEA system. You can still find deals that are within walking distance. If you can't, the ones out on the various loops are not that far, half the price, and you can park at the Alamodome and shuttle over.
One more thing to consider - we KNOW when the convention will be in 2021, 2022, 2023... Book your room this upcoming summer. Book your room for TBA/TODA/TCDA while you're at TMEA. Often you get discounts for early booking or for locking in your rate.