Good.
But:
TX has 19 million people right now over 18 who are citizens.
Which means 19 million people should be automatically registered to vote, like in most other countries.
There's still a gap of 3 million people, because only 16 million are registered.
Maybe not 3 million, if you account for prisoners who are not allowed to vote, but that's probably just a few hundred thousand.
You're correct, according to this Texas has ~250,000 residents in prisons (state, federal, local).
https://www.prisonpolicy.org/profiles/TX.html
