I think "but" is fine. Some people invent crazy rules, but the important thing is to be clear in one's writing. You might like this little book, if you haven't already seen it: "Eats, Shoots and Leaves" by Lynne Truss. It's about punctuation, but it's got a lot of common sense advice and helps one to see what is important in writing.

It is fine to use it in the middle of a sentence. There is disagreement about whether it is OK to use it as the first word of a sentence. To avoid arguments, don't do it. I made some spot checks. I looked at the first scientific paper I found on the PLOS ONE website. I found the sentence "Zika disease is a flaviviral infection transmitted mainly by Aedes spp. mosquitoes of tropical and subtropical areas, but other vectors can be involved in the transmission such as Culex quinquefasciatus." I looked at a report from a government committee and found "The strategy required under subparagraph (A) shall be submitted in an unclassified form, but may contain a classified annex." I looked at the State of the Union message delivered by the President of the United States to Congress and found: "We endured floods and fires and storms. But through it all, we have seen the beauty of America’s soul, and the steel in America’s spine." In short, it's fine.