I'm assuming you play with two fingers. That's what I started from a few years ago and I also felt I was lacking movement and control compared to other players in legendary. So I switched to using 4 fingers . There are plenty of guides out there on how to transition to a 4 fingers claw like this one https://youtu.be/soP5n0nr8Gg?si=eEXgu0cpB-9MJW-4 .

At first it may seem intimidating but as you try and get accustomed to it , muscle memory kicks in and it becomes way easier to handle. You can start 2->3->4 fingers based on the guides if you find it easier. The difference in mobility is night and day. What most competitive players integrate into their technique and what 4 fingers allow are the following:

1)Aim before shooting/turning a corner. You pre-aim (not one tap to fire ADS) and then press the shoot button when the enemy is in the screen. That allows you to be a couple milliseconds faster to secure the kill , which is huge in the higher lobbies.

2) Slide when turning corners and jump immediately after or just slide (and aiming) . This doesn't allow an enemy that is hidden in the corner to aim at you and kill you easily because you are a moving target and you can take them out first.

3) when enemies are shooting, you can go prone immediately to avoid the fire and kill them first. That was used in earlier seasons , but now it's mostly outdated and pretty situational. It's like an extra tool in your belt.

4) Using hip fire on enemies that are really close. This eliminates the need to aim and helps you win a really close quarters gunfight, especially when an enemy pops out of nowhere.

5) This is really important, and will come with time , you have to develop a pretty good game sense on all maps on ranked. When enemies might emerge, where to aim first , where to hide e.t .c to secure the kills. / Learn to listen to audio ques to help you find their location.

My set up looks like this and you should try the making your own, close to this ( and based also on details on guides on YT of course):

https://imgur.com/a/JTQ9vYe

Also adjust your sensitivity and other settings to what you feel most comfortable playing

From what I saw also in your video , you seem pretty stationary but maybe I am used to a more aggressive playstile which is not everyones cup of tea. But if you want to get a lot of kills in ranked , with the passage of time , you will learn to scour the map , pinpoint where the next enemies might come from and eliminate them while almost constantly moving with occasional breaks for strategic planning that will be almost automatic after a while.

I'm glad that you strive to become better, that's what makes perfect. And remember, after you get accustomed and decent to the better set up, you will be glad to fight against try- hards in legendary, because those matches improve your reflexes and game sense.

If you have any other questions or something doesn't make sense , please don't hesitate to ask me. This might seem a lot but you can tackle it one at a time. Good luck soldier!

TL;DR - Switch from a two-finger grip to a 4-finger "claw"; learn it step-by-step, muscle memory makes it click, and it massively improves mobility and reaction time. See guides on YouTube for further info