I've seen a lot of comments on this sub suggesting people lower their dpi to improve their aim. While technically true, it's basically the opposite method you want to use. You want your DPI to be set to whatever you find comfortable to navigate your desktop/UI in game. You want to use the in game sensitivity sliders to bring your game down to a reasonable sensitivity.
A reasonable sensitivity is generally determined by the space you have a available. Coming from Counter-Strike, I have a 45cm(~18") mousepad, and play with what is considered a high sensitivity for that genre: 43cm/360. For reference, that's 1600 DPI at .1 sensitivity in Tarkov.
I feel that setting your sensitivity to be mousepad/360 is the most versatile if you want to use the same sensitivity in all the games you play. It is essentially the lowest sensitivity that allows you to react to something behind you with ease/without lifting the mouse. As such, I believe it to be the optimal sensitivity for Tarkov, as being able to pick a direction and return fire, or run, is essential to survival. While you can do that with a higher sensitivity, you basically end up sacrificing accuracy for no benefit.
TL;DR DPI is for your mouse cursor, sensitivity is for your aiming reticle.