You've got tons of options that let you actually work with math without getting into traditional engineering. The most obvious one is Pure Mathematics, where you work with theory, proofs, and abstract concepts without needing to apply them to bridges or circuits. But if you want something with better job prospects, consider Physics, Statistics, or Actuarial Science - those last two are brutal with numbers and there's real demand for them in companies, insurance, data analysis.
Another path that a lot of people don't know about is Actuarial Science, which is pure math applied to insurance, pensions, and risk. If you're more interested in the practical side but different from engineering, Econometrics combines advanced math with economics, and there's job openings in banks and consulting firms. There's also Computer Science or Computer Engineering (which is different from Civil or Systems in that it's more theoretical), or even specializations in Cryptography and Number Theory if your country offers them.
What I'd recommend is that you check out the nearby universities and see what subjects each program has in the first year. A lot of the time the curricula are similar at the start, so you can begin in one and switch without wasting time. And take advantage now to really push yourself with higher-level math - it won't just help you decide, but it'll get you ready for whatever you end up studying.