Don't use a slip lead, try a headcollar. Maybe Halti's Gentle Leader, a Canny Collar or a Gencon.
I use a Gencon with Flo just for street walking, which isn't that often as she spends most of her time off lead. As soon as she has the Gencon on she walks to heel on a loose lead. Others on the forum have tried alternative head collars and can let you know what they think - some dogs take to them easily but some dogs hate them and are very unhappy. I wouldn't keep using a head collar on a dog that was very unhappy using it but it does take a few attempts to get them to adjust to wearing one.
You could also try different methods for teaching good lead behaviour - I think Jukee Doodles has posted advice along with a link to a video.
I bought the gentle leader from amazon ( much cheaper ) for Scamp who pulls dreadfully on the lead and always wanted to be in front !
It worked first time and once its on he ignores it !
He goes off lead in parks and in the countryside so its not a problem then and he wears a harness to secure him in the car.
Oakley was, shall we say an enthusiastic walker on the lead, yes he dragged me around everywhere, and he will still pull now and again but generally he walks much better these days, which i think is him maturing and consistency from me or my hubby, he knows what we expect from him.
Ok I have three so when it is only me walking the 3 of them on leads, I have to be in control or it would be impossible, but they do try to compete a little, they like to walk in a row.
When I walk Oakley alone, it is a breeze he is much better than he was in the early days.
Just for ref Honey pulls and I am always correcting her. Eevee is always full steam ahead, it really does get better, just stay in control, even stop walking and stand still always makes mine sit, stop and look tome for a command or the next instruction, turning with the lead works well too, let the dog know you are in charge, it takes time and it is early days, don’t worry too much
So time and consistency is my recommendation. Training classes may help too. I hope this helps xx