Any tips on how to slow down a horse coming to the jump, to get a better distance?

Ean back and half halt🙂 More flat work will also help.
Quietly circle before the jump until he waits for you to tell him when he "may" jump. He will learn to wait. Circle after jump again because he must learn to wait for you to tell him when he "may" proceed to the next. Eventually you will be able to string together a course in a relaxed manner. Requires patience but it pays off with no aggression or arguing.
Lots and lots of flat work will fix that!
Lean back and when practicing make sure to do circles until your horse has a steady pace. If that doesn't work, trot the jumps
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