As you know the article of "Schatz" is "der". My problem is this sentence: "Du bist mein Schatz".
I reckon that if I'd like to address a male,
Du bist mein Schatz
is ok but if I address a female it should be
Du bist meine Schätze
I saw some contrary examples and am not sure that they're credible.
As you know the article of Schatz is der.
Correct. The grammatical gender of "Schatz" is masculine.
My problem is this sentence, du bist mein schatz.
As an aside, this should be: "Du bist mein Schatz." Nouns and the first word in a sentence always start with a capital letter in German.
I reckon that if I'd like to address a male, du bist mein schatz is ok
Correct - again, it should be written "Du bist mein Schatz."
but if I address a female it should be du bist meine schätze.
Incorrect: If you address a female, the correct sentence to use would be the same as for a male addressee, "Du bist mein Schatz."
The reason is that the grammatical gender of a word and the biological gender of the lifeform described by the word do not need to match. They can be the same, or they can be totally different.
What you may have seen in use is the form "die Schätze" - but this is not a female form, it is the plural of the word. As the definite plural article in German happens to be "die", just like the definite singular article for feminine grammatical gender, some sentences may indeed be ambiguous if you do not know the forms of the word, but there is no singular form of "Schatz" that reads "Schätze".
To underline this with concrete examples showcasing the correct usage:
(Thomas and Wilhelm are male given names, Klara and Claudia are female given names.)
Grammatical gender doesn't work that way. It's a property of the noun you use. It doesn't align with the gender of someone. Let me show you.
Mein Wagen muss in die Werkstatt. Er ist kaputt.
Meine Karre muss in die Werkstatt. Sie ist kaputt.
Mein Auto muss in die Werkstatt. Es ist kaputt.
It's always the same car I talk about. The car has no gender at all. It's a car. Yet I can talk about it with a masculine noun der Wagen, with a feminine noun die Karre, or with a neuter noun das Auto and even have to align the pronouns in a followup sentence to that.
That is because a noun's grammatical gender never aligns to the gender of someone (or something). And that of pronouns must align to the grammatical gender of the noun they refer to. Only if there is no noun, the gender of someone is used as a fallback. This is only the last resort.
Du bist mein Schatz.
So this works for men, women, cars and anyone and anything else.
Du bist meine Augenweide.
Again this works for men, women, cars and anyone and anything else.
The only exception to this is nouns made from adjectives as they come in all three genders. So they are matched with the gender of the person described.
Du bist mein Liebster. — for men and masculine nouns (e.g. der Wagen)
Du bist meine Liebste. — for women and feminine nouns (e.g. die Karre)
Du bist mein Liebstes. — for neuter nouns (e.g. das Auto)
And no, you own gender does not matter albeit from choice of vocabulary. Women are usually more refined and modest in that.
This is no different from very frequent words wie "das Mädchen" or in older German "das Weib" (which was a neutral word in earlier centuries). You say "liebes Mädchen!" with neuter inflection, it is not possible otherwise. With words that are in immediate contact with each other, the grammatical gender has to be observed (and Germans hardly perceive that as special).
It's different when there is a distance, like when using a pronoun. In older texts, you would have had: "Das Mädchen bekam einen Ring geschenkt. Es freute sich ungemein." Today, people feel uneasy and would perhaps prefer "Sie freute sich sehr", but it still feels difficult (for me). Anyway, when you take up something from earlier text, the real sex of a person may play a greater role in the decision, but not inside a grammatical phrase.
So the problem that you have brought up would also be felt by German speakers if it is about a text. So when a boy talks about his girlfriend and says: "Mein geliebter Schatz saß neben mir ... ??ER hielt meine Hand..." This is indeed odd for me! But the first sentence was ok.
Incidentally, this is a frequent problem in languages with grammatical gender and may find interesting solutions. I read that in Russian you get as a rule that adjective-noun agreement follows the grammatical gender, but already subject-verb agreement may not. Russian has gender forms of the verb in the past tense. They would then say things like: "Molodoj vrach prishla" Here, "vrach" is the word for medical doctor and is masculine gender, hence the form "molodoj" of the adjective. But if it is in fact a female doctor, then by the time you get to the predicate of the clause this starts to prevail and you choose the female form "prishla" insted of masculine "prishol". So it seems to be a general phenomenon that grammatical features dominate over meaning features when it is a very tight grammatical construction.
I'm a male and my born, raised, lived and native German aunt always called me "meine schätze." when I was growing up. I can't elaborate any more unfortunately, or explain in detail as well the others above due to the lack of my understanding of the language, my Deutsche is... let's say, less than conversational. I posted to say a naitive life-long and still-speaking German, at one time called me (on many occasion), "Schätze". :3
meine Schätzeis plural