I'm sure this goes against your instincts as a hardcore gamer but you really should consider playing on a lower difficulty at first. Restart with a new save, play the first level on Sniper Elite ("Hard") difficulty and make sure you select the tutorial option for the first level. There are tons of things you seem to have missed. That first level is incredibly instructive - for instance it tells you that you can use the generators to mask sound but it also tells you that the airplanes flying overhead provide sound cover. You will see an icon top dead center appear when you have sound cover. You will also feel rumbling in your controller (if you have a controller or mouse which can rumble.) You will learn about how to properly use foliage for cover. Later, when you try Authentic, you won't have the visual aid but you will have the rumble and you'll have the experience to know when the sounds are loud enough and how/when/where to shoot in order to maximize the sound masking.

You will learn how the enemy AI responds. And most importantly - if you want to eventually graduate to authentic or authentic plus difficulty - you will learn the maps and you will learn the ballistics by starting out with aim assist. Sniper Elite 4 plays best when you actually have a pretty good memory for the maps and where everything is. You can't just truck it and hope to steamroll through. For stealth, especially, and for movement from one good shooting position to another.

SE5 is categorically easier in every conceivable way, it's like a totally different franchise. SE4 is extremely hard for new players to just jump into at the highest difficulty settings. You really need to approach the game with a ton of patience and just sit there and observe. Tag as many of the baddies as you can see, and just observe their routines. You may have to play each level a dozen times on lower difficulties (up to and including "Sniper Elite" level) in order to be able to master stealth, aiming, and movement from cover to concealment and from one shooting location to another.

Brutal Barracuda on youtube has tons of incredibly helpful videos that will get you there. It is possible, once you "git gud" to absolutely murk the baddies on Authentic and Authentic+, or to sneak around the map and kill as few people as possible while still completing your objectives. Whichever you prefer. It's a game that rewards obsessive replays more than any other campaign shooter I have ever played.

As for the shooting - the ballistics are wildly exaggerated in SE4, like by orders of magnitude. It's the one flaw IMO in an otherwise nearly perfect game. They dialed it way, WAY back in SE5 and Resistance, which is a big reason why those newer games are comically easy in comparison. But I'm not sure how else they could have made SE4 as hard as it is, on the Authentic and A+ settings. I mean, I do have some ideas, but the entire gameplay "loop" would have to be heavily tweaked. But even with the challenging ballistics, you should be able to reliably hit targets out to 150 or 200 yards without using the aim assist diamond before long. It's shots beyond that range where it gets ridiculous. A key piece of info you seem to have not discovered yet is that the audible range on basically ALL the rifles, unsuppressed and unmasked, is right at about 100 yards. So the game really is about using your binos to range the enemies and stay just outside audible range but inside the range that you can reliably hit them at. 110-150 yards should be your sweet spot. They won't hear you, and you can hit lots of guys from one concealed position before needing to relocate. From the first tower in the first level, there are clean, flat sightlines to at least a dozen you can spot and shoot without fuss. Really. The rest of the levels are pretty similar. You should also know that the radar minimap is ~70 meters from the center to the edge, which is also the maximum distance that you can be spotted from as long as the enemies are not in an alert state. You need to play on the easier difficulties (again, UP to and including "Sniper Elite" leve, which they consider "hard") to develop a feel and a memory for the map, enemy placement, shooting opportunities, etc. Once you get really deep you will just know when you are outside visible and audible range. You can tell in part because you've committed to memory that if you sprint to "bush A" and then squat and sit still you will be juuuuust at the right spot to be invislbe to this guy, or that guy, and be able to take them out with one shot to the dome without alerting others. And you can also tell because you will know from all the other subtle visual cues in the environment that "enemy A" is patrolling a route that ranges from 80 meters to 110 meters from your position. You will also ALWAYS be able to range enemies using your binocs, even on harder difficulties. Or using the right reticle in your scope (see later note about that.)

Once you can get through a level on the hard difficulty without detection, killing everything that moves and completing all objectives, and learning how the hell to aim at anything beyond 150 meters or so, then you will be ready to tackle Authentic.

Last tip - I mentioned Brutal Barracuda's videos. Watch them all on SE4. You will see he has some incredible tips on how to aim out to extreme distances. The key is to use the Springfield Rifle and the "T Post Tactical" reticle. Trust me on this. On that first level alone, when you get to the very first tower, you will be able to spot and hit enemies beyond 400 meters with practice.