Divorce vs Separation in Texas

My wife and I might not stay together. We have been married for 12 years, and living together in Texas for 10 prior to being married. We have no kids. She has not been employed in over 5 years and might have only been employed a total of 3 years over the 22 or so years we have been together. We do not have a pre-nuptial agreement.

If we decide it is not going to work out, I would like to protect my future financial earnings form that time as being only mine, but I would like to stay legally married for some time to keep her on my health insurance. I do not want her to be 50 something and jobless with no health insurance. I will be financially supporting her for a pre-determined amount of time for her to get her feet under her.

Would a simple written contract between us splitting up our assets as we see fit as well as any financial obligations I would pay for a pre-determined amount of time and stating that any future income is not community property be legally valid in any way?

If not, is there any way to stay legally married for a time to keep her on my health insurance and protect my future financial gains. Would this require a legal separation? Even if we stay together, I would like to protect my financial assets going forward. Is there such a thing a a post-nup?