If you like such systems, backend systems in investment banks is where its at. Most of my experience was at Goldman, but similar systems exist in Morgan, Deutsche, Citi, UBS, Bloomberg or old PE firms. Look for C programmer careers in trade publications like efinancialcareers.com, or work with Huxley, Selby Jennings etc. Combination of C/C++ and certification in Q/kdb - will keep you employed pretty much into your 60s. Those languages are intrinsically hard & imo a real pleasure to code in. Well, I liked it.