What you learn in Excel will be transferable to any other language, but a language like Python will not be immediately transferable to Excel. This is because Excel is based on more mathematical constructs (functional declarations / formulas), while python is like giving orders to the computer exactly how to do stuff (imperative programming). Don't worry, you may need some time to learn the distinction, but it's there and it's real. In a way, building with Excel is like playing with lego. You see connections and build prototypes, that may help you later, and it's more fun. Coding in a real language though, requires that you know more exactly how to build something and also learn alot on how to use that language and libraries.
Excel's got an undeserved bad reputation among programmers. It's usually accurate enough and you can weed out quirks and bugs, just like any other language and framework. Fixing fundamental flaws is really part of programming, since your tools are never perfect to begin with. What it'll give you though, is more intimacy with the numbers and formulas, which is good in the beginning.
It depend on your temperament, capability to learn and what'll work for YOU. In the long run, Python will make you able to program most anything you need, while Excel may make you more aware of what you actually need faster. So you may learn faster and accomplish more in Excel at first, and this might be good for you in the short-term, both to program and to specify your thoughts into instructions for the computer.
Using something like Python is like driving a car instead of a bicycle, you'll need to learn more, but can get more done later. However, there are other languages too. C# is not too bad and can be used in Ninjatrader, where you also see charts directly. So it's not really either-or, but you can do both, which of course may take more time and work than you originally planned for. In fact, the options for programmers are often so overwhelming, it's hard to get projects properly designed and done, until you know exactly what you need and how best to accomplish it using existing tools and libraries, or writing your own from scratch.
I know we all want to rush things, and it's natural, but these things takes time. What I said about automatons upping Murphys Law is no joke. It'll take ALL instances of signals, even those a human would pass on immediately. Not sure how you've managed to backtest everything already if you aren't a programmer, and there are all sorts of biases that creep in, ie. from your own selections or surviving instruments.