Others have chimed in but I'll add my 2 cents.
First off, they will frame this as school choice, as if they give a shit about a poor inner-city kid not being able to go to private school.
1) It's not about a private school being better, it's that they can send more kids to religious schools with taxpayer money but also reimburse families who are already wealthy enough to send their kids to those institutions. Basically, "my tax dollars shouldn't go to public schools where Jesus is not taught". The current Senate bill does have funding for public schools. The breakout is $4.5 billion for about 5.5 million children in public school and $1 billion to about 100k private school students. The public school funding is being held by Abbott and gang until vouchers pass. The Senate bill also acknowledges the cost of the vouchers will be around $4.5 billion by 2030. Gee, I wonder where they're going to take funds out of to get to that number.
2) Nobody can answer that but them.
3) I personally believe you will see a ton of for-profit "schools" pop up where people will try to get some of these funds. The private schools do not come with any testing requirements like public schools, so someone is going to see an easy way to make money off the taxpayers. But yea, no benefits to rural families really.
4) I think eventually the income limit will be raised and the costs of the program will continue to spiral out-of-control.