said you a lot
but I find something worth adding
First of all - "click the behaviour, reward the position" - I found it extremely important in heelwork. I reward Una using my left hand and I'm trying to give her a treat in a perfect position.
Second - rewarding only in a sitting position when the handelr stops is a good way for some dogs - but not for all (I do it like that on this video:
o, and there is "down" cue on the end
Third - try to do "brain wash" about heelwork - use your dogs favorite treats, make your session extremely short and extremely easy - one step - c/t. Let your dog to love heelwork!
And last but not least - when you are going faster in becomes easier for a dog. So - go fast. Walking slow it will be the next step