Pyrenees teenager starting to guard us? Advice appreciated

Seeking advice, reassurance, and recommendations!

We adopted our sweet Pyrenees mix pup, now an ~18 month old adolescent "teenager", back in February. She adjusted to city life quickly and has been super friendly and well behaved, even responds well to basic commands. But recently she's started showing some aggression towards other dogs, in particular when other dogs are near her humans (my partner and I), so we suspect this is her instinctive resource guarding starting to come through, based on her age and because she has had time to bond with us and may now see us as her flock or pack.

We knew this was a possibility given her breed, but the inconsistency and unpredictability is especially confusing, alarming, and upsetting. Most of the time she is a happy, playful pup, who loves to run around with other dogs more than anything! Sometimes dogs come near us or jump on us and she doesn't seem to care, but other times and seemingly more often with Australian Shepherds and/or other female dogs, she goes full Guardian Mode with very little warning: lunging, flipping them onto the ground and snarling until we pull them apart.

We take her to the dog park every day, and she regularly goes to daycare and camp-style boarding without issue, so would be devastated for her to miss out on playtime and socialization (surely that would only make it worse!) but we worry now about putting her and other dogs in that situation. Our plan is of course to get professional training (NYC recs welcome!), but we haven't ever had a trainer and I'm worried that it won't ever be "enough".

Has anyone else experienced this sudden change during adolescence? Did anything work? Is all hope lost for our pyr's social and exercise time? 🥺💕