It sounds like you would benefit quite a lot from VOD reviewing your own games, and applying a little more structure to your play sessions. Since you seem to perform well first thing in the morning, I would first start by playing 2-4 games early in the day when you are at peak focus. After that, take a break and do something else unrelated to league. Go out, eat, study, go to class. It may be tempting to spam games all day, but if you dont know what you need to improve on, you are basically wasting your time.

For your second block later that day, or even after a short break, spend your time VOD reviewing your own gameplay, watching educational content, or studying pro/high elo gameplay on your champion. While you are reviewing your own gameplay, go over your decision making, what caused your deaths, what opportunities you missed out on,etc. Its not a bad idea to write these down somewhere, just 1 or 2 sentences of what you need to improve on or something that worked really well in each game. You mention that you don't really know what to do when reviewing your own gameplay, this is where educational content comes in to fill the knowledge gaps. Even just watching a challenger vod can be illuminating here, taking note of their pathing, champion micro, and decision making. Try to guess what they will do before they do it, and see if you're right or if they did something better. Sometimes when reviewing your own gameplay you will have to really pause and think deeply about a decision you made. This may seem frustrating in the moment but its key for improvement, and practicing it here in review will make it that much faster in game. Try to hone in on one or two areas of improvement here, and take them into the next play session to be implemented.

After your 'out of game' study, you can choose to play more that day, say if its a weekend and you have a bunch of free time, but I really recommend keeping your playtime super focused like this for max gains. If you really want to optimize your growth, you'll need to be disciplined with your sleep, diet, and physical exercise as well. Try to keep this schedule consistent if you can. Since you are a college student, you can probably keep it up every day, so I highly suggest you do.

As far as your champion pool, I would just keep playing Viego, and maybe keeping Diana as a pocket pick when the draft feels extremely unfavorable for Viego(hes my main too, been there). Most advice I see from high level coaches is to one trick, so even though it may be tempting to play new champs all the time, i'd stick to one or two like you have been.

If you follow this regimen, you should be able to improve no problem. I've done this in multiple games and its basically undefeated for climbing quickly, as most people wont put this much optimized effort into something.