If you’re gonna use a jarred sauce, go for one of the more top shelf brands. They don’t load their sauces down with sugar. I like Gia Rosa. Season to taste (even the good sauces could use some extra garlic or salt & pepper) Add a couple splashes of Worcestershire sauce. Not enough to actually taste it. But a small amount adds complexity. Add a little red wine. Again, for complexity. And finally.. let it simmer for an hour or two. This gives time for the natural sugars in the tomatoes to caramelize. This will add even more complexity, as well as adding a bit of sweetness without a bunch of unnecessary sugar. This step alone will improve just about any tomato based sauce. It’s all about layering flavors.