While I have a solution, my brain only works in Swift. I think this (untested) solution would be accomplished through a Swift Subquery using NSPredicate.
let predicate = NSPredicate(
format: "SUBQUERY(
embeddedDogList, $dog, $dog.status == 'healthy' AND $dog.age >= 3).@count > 0"
)
let results = realm.objects(User.self).filter(predicate)
So we least know it can be done. Perhaps a flutter/dart expert can chime in.