First and foremost, you need to work on doing more with what hits you get.
Very often, you'd connect with a 5B or 5C and then just leave it at that when you could have gotten much more damage off of them. Try to find out what hits in what situations can lead to what combos. Also, refrain from using Hell's Fang like a command dash, and also try not to finish Hell's Fang (Don't do the followup basically) if you whiff/get blocked.
You have heat, use it! Rapid cancels are a must, either to prevent punishes or to extend combos for huge damage. Learn some rapid cancel combos ASAP, they're a huge staple in almost any characters game. Blood Kain can either be your saving grace or biggest mistake in the world. Use it to scare people. Finally, Carnage Scissors is pretty mediocre as a super but it can prevent bursting near the end of a match.
22C is one of Ragna's best moves. Abuse it to no end. It scares people, it hurts people, and it sets up for a lot of punishment. Incorporating 3C and 22C into your game is a great way to improve.
Something I saw you doing a lot was using 2D. Yes, it hits low but it is exceedingly unsafe even without IB in the picture. 2B and 3C are much better for Ragna's low fix if you're close enough to use them. 3C in particular sets up for 22C on hit and can be jump canceled on block.
Finally, use 2C and 5B to fish for most of your hits. 5B is just a great, mostly safe poke all around unless it gets IB'd and 2C has counter hit stun comparable to Hell's Fang (But it's much safer). 5C is only worth doing if you're very confident it will connect, on IB it's extremely punishable.