This is a good start, and it is also a perfect snapshot of where most spreadsheet learning gets stuck. You built data validation lists, used conditional formatting, created pivot tables with month-over-month change, added slicers, and made charts. That is a solid technical foundation. But the real question is not whether you know the functions; it is whether the tool is working for you or you are still working for the tool.
The moment you described, realizing DSUM is outdated while your course still teaches it, is the moment most users never get to. You saw the gap between what you were told to learn and what actually makes sense. That instinct matters more than memorizing SUMIF. The problem with traditional spreadsheet training is that it treats the software like a static manual: here is the function, here is its syntax, now apply it. It does not ask whether the function is the best way to get an answer or whether the answer itself is the one you actually need. You already started asking that question, and that is what will separate your skills from someone who just follows a tutorial.
What you could have done next is shift the focus from what the spreadsheet can do to what you want the data to tell you. Conditional formatting on sales numbers shows you highs and lows, but it does not tell you why they happened. A pie chart of regional sales shows proportions, but it does not show whether one region is growing faster than another or whether a single customer is driving the numbers. The next step is not another chart type. It is asking: if I had to explain this data to a manager in thirty seconds, what is the one insight I would lead with? Then build the spreadsheet to answer that question directly, not to display every available calculation.
You are on the right path. Keep questioning the tools you are taught. Keep building things that answer real questions rather than just demonstrating features. The spreadsheets that matter are the ones that make a decision easier, not the ones that look complete.