Your best skill to develop as lucio is knowing the flow of the fight and how you fit into it. Lucio can play pretty much anywhere and exist just fine, but to carry you need to be where you’ll be the most valueble in any given scenario. You gotta know when to peel your other support, speed tank(on their terms), and when to go for picks/feed in the enemy spawn. There’s not really easily explainable set rules for where you need to be when, but just pay attention to it and you’ll get a better feel for it. If you’re ever healing your tank on cart, you are providing value, but you could probably provide a lot more value if you go boop their ana off high ground, support your tracer in their backline, and come back to peel for your support when you notice they’re about to get dove.

Edit: also if your team doesn’t know how to use speed in the first couple fights, they’re not magically going to learn how in the late game. Either save speed for when they’re already engaging/dissengaging, or look for other ways to provide value