You can beat Pre-mades.
You can beat pre-mades with a mediocre team.
Sometimes you’ll get stomped, but no matter who you are facing, sometimes you’ll get stomped.
While I wouldn’t be against a seperate queue for pre-mades vs PUGs, I think that it would just increase queue times again and upset people.
If I could provide any suggestion, it would be learn to play as one of the following characters: Phoebe, El Dragon, Thorn, Rath, OM (there are a few more).
These characters are all hard Carries. If you manage to keep level, and play the objective, you can single-handedly keep your team afloat.
Speaking of which, the only other suggestion I would give, is that when up against a pre-made, focus on playing the objective above all else. You are likely going to lose fights early on, because they can apply group tactics and you can’t. If you play the objective, you will start to out level them, when you can start doing real damage mid-late game.