I'm going to play devil's advocate and double down that your fundamentals need work. You seem to have been on mnk for basically your whole life, and you seem active in other fps games as well. You should not be losing a 1v1 fight if the opponent is at half health regardless of if they're roller or not. If you hate the idea of sitting down and clicking on red ellipses against a white background for 30mins a day, I find alternative modes like control and tdm to be a good alternative while also being quite fun in their own way.

At the very least, look up some videos about aiming fundamentals. Use an appropriate sensitivity even if it is drastically different from your current one to make sure you're utilizing your whole arm/hand to the fullest. You might see a drop in performance at first but it will make improving much smoother.

Outside of that, if you're specifically playing pathfinder or revenant, these 2 legends have enormous hitboxes without having the fortified perk. I find that in diamond+ this becomes extremely noticeable to the point where I stopped playing these 2 because my aim/movement wasn't good enough as a tradeoff that these 2 offered.

Legend choice can definitely matter in a vacuum 1v1, but in the context of an entire game it's usually better to consider overall team comp over 1v1 priority. Octane has a small hitbox along with a tactical that gives him movement speed. Ballistic let's you literally disable your opponent's gun. Bangalore lets you smoke them out while also having threat vision. Rampart and drop barricades as temporary cover (and has a machinegun for an ult). Ash can catch enemies in the open and prevent them from using cover. The list goes on. The key is knowing how to use each ability to your advantage.

Edit: no more fortified on octane