Straight jumper here but with a little experience in eventing too.
You said it yourself - when both you and Rebel feel confident and comfortable. You should be able to ride an entire course at one height, including oxers, doubles and triples, tight approaches etc. before moving on.
Overfacing a horse, even one that willingly goes forward to the fence, can create a stopper. And that can sometimes be really hard to fix.
My first real jumper was a 4' horse that did the 5' on heart. Even overfaced he would try to jump anything. And I guess I was too young and inexperienced to realize we were horribly overfaced. We had a few crashes, a few injuries. We went on to a couple GP at 5' and we won a high jump at 5'9". But he was basically a 4' horse.
If you ride to a fence feeling panic your horse will too. Take it slow, have fun. You'll get to the bigger fences when you are ready.
I've got another jumper but I've trained this one myself and brought her along very slowly. If we rode a course and she hesitated or stopped I'd drop the fences back down and ride courses at that height until I felt she had her confidence back.
Don't rush it. You or your horse could be badly hurt. It's supposed to be fun. No panic allowed.