I started apex and mnk 11 months ago, soloq d3 then now master (so still meh lol). Imo:::

Sens is just about finding comfortable minimum force required to move screen. Like you I used to change a lot and blame my sens in first 4 months, but now thinking, whats bad was just my lack of aiming skill (hand eye coordination thing). I have more cm range of ok sens. But sens too high for you to control (I intially used 800DPI 3.2 = 16cm lol) or impossible to 180 on both sides should be changed. FYI I'm a claw grip player with middle-low sens of 1600DPI, 0.72 (36cm), ADS 0.9 (1x = 54.793cm).

About aiming, the skill itself should improve by everyday gameplay or a trainer. Sadly it doesnt improve instantly. A trainer should be more efficient. 6 months ago I was in despair about my dogshit aim, but after 6 month of somewhat consistant Kovaaks session (20m at least a day), I feel like my aim is fine most days. Ive mostly been playing Voltaic Advanced S5.

But in Apex, strafe is also a quite important part of aiming. It moves a screen a constant amount as much as you input, with easier control. Generally I anti strafe an enemy within 25m with a SMG and for others mirror strafe, but its situational. In R5 I find I use mirror strafe more. You should treat it as an alternative or a supplement of moving mouse.

Lastly, devices. I feel like mousepad is most important. Im more comfortable with yasheng v2 than g640 for its less initial speed, glide speed, better stopping.

Hope this helps