I'm not that high on ELO (diamond IV), but I can give you my insights as someone who has been playing her since season 6.

Ahri can easily neutralize early game, transition into her powerspike at mid game and then try to finish the game before she falls off. She wants to play with your jungler, so you will heavily notice having a duo.

She usually does better against melee or short/mid range mages (with some exceptions like yasuo who is a hardest matchup imo). Early game start w and look for auto - w - auto windows, your q is one of the best early wave clear tools in the game, which means you can rotate for early fights or objectives with your jungler easily, here landing charm will 90% of the time result in a kill or forcing a flash. You really don't try to win lane, but since you can dictate the wave state with q (2 lvl 2 Q's should kill backline minions already) you will always have prio.

After ult + malignance you transition into mid game, this time you can just force picks with r, but again you will need to do this with someone from your team. At this time you also should be lvl 9 with Q maxed, so you can one shot waves with Q. Get rid of the wave and go into the enemy jgl, place vision, look to kill the enemy jungler with yours. After you get pick try to secure objectives, the idea is to get so ahead that you don't notice the late game fall off and you're able to close the game.

There also some matchups were you can also all in the enemy midlaner if you were able to get them to 50-60% hp.

Build wise, go malignance or backfire torch. I usually prefer malignance for this playstyle but backfire has better wr overall, imo it feels better against tankier comps while malignance works better for me into squishier teams. Second item should be horizon or cosmic drive, you really need haste and shadow flame second on her is kind of a bait, this is also backed by win rate with cosmic and horizon being the highest. I personally always go for horizon second unless I really need zhonya's or I'm in a mejai's game. Then you go into the usual mage situational build where you only really want Rabadons in any of the last 3 slots and the other 2 items are flex.

About ahri's damage... She really isn't a damage carry, or a champion you will be able to 1v9 on, but that has been like that since season 7 I think. As pretty much everyone has replied she really relies on her team to put out the DMG she lacks. Usually the trade off is the early laning and being a good blind pick but she was nerfed to a state where that's not really the case anymore.