Cook a lump of liver with garlic in water till cooked, then cut it up into really small pieces bake until hard in a low oven, bag up and freeze get it out as needed
I have always used this to train dogs all breeds and they all like it once had 16 dogs mixed breed following me across a field and back to their owners without one stopping for a sniff.
Another thing you should try is doing ringcraft off lead.
It makes dogs use their brain and focus the rule with this training is that other people do not touch him if he goes near another dog you just block him and use your voice to get him back.
And number one rule is do not stuff food in a dogs face it’s just a reward and should be used as such.
Dogs that can’t focus in a ring are usually caused by face feeding so the dog does not really understand what you want which is focus at all times