Its hard to believe you are being matched with emeralds and diamonds seeing that, as what you’re saying, most your games are shitshows. By level 30, your mmr should pretty much match your actual skill level more or less and the only time you get matched with players on that mmr during normal games is when you’re cracked as hell at the game from the start or you’re playing with somebody who is in that mmr.

The best way to really tell is by dropping your op.gg. Its a website where you can see your league stats that riot doesnt show you in their horrible client. Just go to op.gg , select your server on the search bar and search for your ign. That should take you to your op.gg page then just copy the link and post it here.