That is an extremely good erg score. There are a few girls on my team who are sub seven but 7:30/20 is really food especially since it’s winter now and you have time to work even harder at it. Awesome, dude.
Well, slide control is a big part of it so that’s good to work on. Another thing you can try is creating a mental plan for yourself before each erg piece or race; this way, you can make sure that you won’t explode into the piece and become drained midway through and you can keep track of when to increase your power and it will help your endurance. Increasing your endurance is a big way to get your split down and power up. Also remember to get in a good warmup and a good cool down, this might sound trivial but you can increase your power by keeping your muscles warmed; otherwise they can lock up during or before the piece. Other than improving on your technique and creating muscle memory, the best way to increase power and decrease split is practice. Do practice sprints, steady states, and learn to find your rhythm. Talk to your coach to see if he/she recommends anything specific to you, but overall work on getting your split lower and lower with each piece. Set goals for yourself and as you meet each one, create another, like a longterm pyramid.
Basically: technique improvement, endurance improvement, practice, and goal setting will be your best friends.
You can also look here for tips you might find useful: