if you want to main mid and climb out of low elo then your best bets are:
TF: this is probably the easiest champ to carry on cause of easy waveclear and a point and click stun. Its really easy to keep from falling behind (gold passive, wave clear on Q and red W) and once you learn to use his ult effectively there should be nothing keeping you from a 55%+ win rate.
Ahri: this champ is safe and has huge burst. learn how to play safe and use her ult for offensive plays only if possible. If you get ahead on ahri it will be very hard for the enemy to lane versus you and it should be impossible for them to roam (without the threat of you engaging with ultimate).
Syndra: one of the most punishing champs in lol. You're probably dead if the enemy jungler can gank you from behind, but that should never happen when playing syndra. Be extra aware of your side brushes. Basically she is just extra long range and high damage. land a stun, Q ult and your target should be dead.
Jayce: Jayce mid was once feared by all. now he got nerfed a little bit, but if you can play a good jayce then there is nothing your opponent can do in lane. you will most likely out poke them and have a better all-in. The good thing about jayce is that he has the ability to play super aggressive like ahri but also has the ability to sit back and clear waves/poke with his ranged Q.
Special note for Aurelion Sol: I actually think that nobody plays this champion in silver or gold. This champion is probably really bad versus the right player. you only really see it in Low bronze and high elo plat+ games. I personally ban this champion in every ranked game where I'm mid. The strengths this champion has are that its 1. buggy as hell and 2. not everyone knows exactly how to counter its playstyle yet. I would characterize it as "passive-aggressive." I honestly think that if you want to climb to an elo that is actually beyond your skill level, then you should learn aurelion sol. I cant even place my finger on exactly why he is op, it really must be his ult and teamfight ability but also his manaless waveclear and insane roam potential. I also think that the fact that stuns dont stop the damaging orbs from moving is pretty op (his main source of damage is not halted by a stun). Maybe I'm just bad. but I think that if you main this underplayed and secret op champ you will probably win a lot of games just because the enemy players will have no idea how to play versus you.
I think the thing to take away is that safety is op in the mid lane. lanes are very often won by forcing the enemy laner to make a mistake and overextend. The thing about assassins is that they usually lose lane by default against "safe" champions if they cant score a kill in laning phase because theyre generally bad at wave clearing and will have to rely on roaming and ganking for map pressure.