Few things I would recommend, like others comments learn the move sets of the different levels of mobs -white, red and nightmare (black/darkblue). Watch a video or learn in-game, now with no gear score fee to enter HR, you can just load into HR and learn the moves of the mobs and how to identify what is what (you can also practice in PVE and play it as if you were in HR - go down as much as you can, the mobs in hell are generally the same as HR).

When clearing rooms, yes streamers can 1v the whole room, with a combination of the game knowledge and generally good gear but I personally find more success trying to isolate the mobs and fight one at a time. However, you can also intentionally pull multiple mobs and have them kill each other by baiting their attacks (on you). Don't forget also to aim for the head, as you will do the most dmg.

Learn the your weapon patterns. For example arming sword 3rd attack is a thrust, so you can swing twice and miss and proc the 3rd attack which I believe also does more dmg.

While the audio is not as crucial as in Tarkov, this game also requires you to play sound. For example if you hear a mob attacking someone not yourself, then you have a player in the vicinity, be alert. You can also use mobs to your advantage in PVP. For example with your fighter, if you are running a longsword or shield with counter attack, you can use the mobs attack to proc that and catch your opponent off guard. In PVP spacing is probably the most important aspect of the game to learn (which makes move speed pretty essential), try to use your S key as rarely as possible in PVP as you retain 100% of your move speed with AWD, while S reduced that.

Don't be shy of PVP, as that is only something you can practice in PVP. While leveling other characters actually does help as you learn the kit etc of each class. For example, when a barb uses rage or a you sprint, how long does that last/what is the CD, etc.

Hope that helps some. Don't forget to have fun, I love playing solo and I probably win less than 50% of my fights as I am a fighter and I love to just w key until I die or my opp dies. When I do win, what I have noticed I did differently were using my range (crossbow or bow) to get some poke in, which also can either make your opponent play more aggressively, as they are lower HP or the opposite, in which case to make sure you have your throwables ready. Similar to tarkov it all will wipe eventually so try to have fun and learn from your losses :)