I'm playing the GoG release of Substance, and the first person sensitivity on the aiming is inconsistent, which is bothersome. In some cases it's so sensitive I overshoot the enemy, in others it's so slow I can't even track the enemies at their walking speed. The game also lacks the pressure sensitivity it originally had, so you have to release the fire button to shoot with handguns. But that isn't the case with rifles. They start shooting the second the button is pressed, meaning you can't actually aim in first person with the rifle's model on the screen. And there are certain walls that when you try to peak around the corner the controls invert because the camera angle changes, which then pulls you away from the corner or sometimes even the wall itself. This has gotten me spotted multiple times.
The ADS aiming is like that on purpose, it's a common way of making aiming not trivial by not being snappy and humanely unnatural like modern games, I think it's totally fair if you don't like it, I don't necesssrily love it but I don't mind it as much. I don't consider this one a skill issue, just down to taste .
I don't know how they ported the game to PC without dealing with the pressure sensitive buttons?? That's a massive oversight if so, you can't do some VR Missions if you can't aim-but-not-shoot with rifles. The Xbox ports had tweaked controls to address this issue so I'm surprised the PC release didn't (well less surprised as I hear the PC release is garbage).
About corner view, the best way to move while hugging a wall is using R2/L2 to move left or right, this should fix your issue of accidentally leaving cover.
You indirectly mentioned a very real issue with the game which is the strange way moving near to cover is handled in the game, I'm not a gamedev so I can't explain it in proper terms, but it feels like the friction values or whatever aren't set up well, if I want to get into cover I'd have to move at basically exactly 90 degrees perpendicular to the wall to successfully get in cover, or I'd just miss the wall and move past it, or keep moving parallel to it. Moving the sticks in such a precise way is something that's essier said than done. The D-Pad has its own issues so that's not a good alternative. In general there is an issue with player movement in the game
Raiden is straight up an upgrade over Snake gameplay-wise, his roll is better than Snake's, and you can actually use it to jump between gaps unlike Snake. I think you only started having the issues because the Tanker chapter is short so it's possible it wasn't enough to encounter the issues, while you play Raiden for the rest of the game.
Finally, if what I wrote sounds excessively deep, blame it on the VR Missions. 30 hours of VR training and I feel like some kind of legendary mercenary...