ST. LOUIS — In many ways, Essence Group Holdings Corp. is a homegrown health care success story. Founded locally, it has grown into a broader company backed by a major Silicon Valley investor. Essence now boasts Medicare Advantage plans for seniors with some 60,000 members in Missouri and across the Mississippi River in Illinois. It ranks among the city’s top 35 privately held companies, according to the St. Louis Business Journal. Market research firm PitchBook Data values the company at over $1.64 billion.