Foolproof leash training method? Help 😭

I have a 7 month old mixed breed (23 breeds, mostly herding and companion) and she’s great. Crate trained, potty trained, all good things. The only thing we’re stuck on (other than the playful mouthing which gets really frustrating) is leash training. We’ve tried SO many things, and the only thing that has helped is a front clip harness. We’ve tried practicing indoors, treats for leash slack, consistency, all of it and none of it seems to stick. She zigzags all over to be sniffy which is fine sometimes but she never really turns it off and the leash seldom has slack. Does anyone have like a foolproof slack leash method?? She’s smack in adolescence and it came at us like a truck so we’re struggling 😭