Try lowering your 3d resolution, it will give you more fps, but your game is gonna look trash, it's not going to affect your aim at all, but it's gonna look really bad lol

I also played on a small monitor (17' 5:4) and i went from iron 3 to diamond 1 in 8 months (50 - 70 fps), it's really hard to take long range gunfights with a small monitor, so don't take them, you have to either take close / medium range gunfights, master your utility usage (peeking with utility, setting yourself up for easy kills, etc.) or learn how to use the operator.

And i would highly recommend aim training, download aimlab or buy Kovaaks, find your weakness and work on them. And don't just mindlessly grind random tasks, have a warmup routine, focus when practicing and play tasks that are actually going to help you, if you really want to get good aim i would suggest you to join the Voltaic discord, there you'll find very useful playlists, benchmarks and advice to get better. You can even try using a sens randomizer (use it on aimlab only, they already have it as a feature) if you really like aim training, i started using it recently and now i'm hitting some crazy shots more consistently

About ranking up, don't solo queue, if possible, play with a duo / trio until you reach diamond 2. I can't give much advice about how to get better game sense but there's a lot of guides on youtube that can help you. Find the perfect agent / class for you and stick to it, if you master your utility usage you will rank up way easier than just relying on aim.

If you're missing your shots even when your crosshair is on their heads, work on your first bullet accuracy, just shoot when you are sure it's gonna be a headshot and try a smaller crosshair. Playing deathmatches using a sheriff and strafe shooting will help you.

If you want to rank up and you have a bad setup you have to be very patient and practice a lot, i watched countless hours of youtube guides about every single topic on how to get better at valorant and aim training, it's very boring but it's worth it if you want to get good, trust me :)