I'm sure the OP knows that ultimately it's her decision, but I *think* she wants to know what we would do if it was our hair with the facts she presented (OP please direct me to take a seat if I'm wrong lol).
If it was me, I'd try to let go of som e of theemotion behind the decision, and examine it from a very clinical perspective, starting with a review of the reasons you named:
1. "I'm itching to relax"- sounds like if all things were neutralized, this would be your preference. Something to think about.
2. "I have a sensitive scalp"- like Kurly Q said, I think this is something you need to seriously consider before you relax. I can't imagine relaxers would do anything besides exacerbate this problem.
3. "I think [my breakage is] due to TX heat and hard water"- you should probably find out exactly why the breakage is occuring before you relax so that you don't bring old issues into a new situation. Maybe start with washing your hair with distilled water, and see if that makes a difference in breakage.
4. "I want to start bunning but all I know how to do with my natural hair is braids and twists"- It is possible to bun with twists and braids, right? When I'm lucky enough to have hair your length, that will my protective style of choice....my hair is WAY too thick to bun loosely, I can't even imagine it. But I digress..... you can teach yourself (via YT, hair boards, blogs, etc.) how to style your hair better. Maybe this would be enough to make you wanna stay natural longer.
My opinion? I think you should address these issues for a few months first and if there's no improvement, or if you're still "itching" to relax, then you should just go ahead and do it.