Again, what are the specs of the parts you have. It's important because maybe some of the stuff you already have may be plenty good and not need to be replaced or upgraded. Example, if you have let's say Saint m810 brakes, they may be previous gen and all but still are top of the line and definitely not top on a 'should replace' list.
Ok, from what I've seen, the first thing I'd replace would be the wheelset, pinned rims aren't as durable as welded rims. After that, look for suspension, replacing the low end domain and kage. But honestly, these are mybe the most expensive upgrades and will definitely cost you much more than you think. A good wheelset is about 1000$ even if you build it yourself and you're looking at at least 1500$ for another fork and add around 500$ for a proper shock. Buying a new bike woth plans to upgrade components right out of the box is a mistake I'm afraid, you'll end up spending much more than buying the bike specced as you like the first time. For now, keep it as is, replace parts when they break.