Thank you.
I think I know 'should need to' meaning. But it's hard to explain with my limited English skill.
So when I say 'should I have' does it have a similiar meaning of 'Do I have to ?' or maybe stronger like 'Do I really have to know more?'
No, it means neither of those things. I'm pretty sure you don't understand these sentences. Most native speakers would get the explanations wrong, because the meaning is rather complex.
"Should I have to ..." does not mean "Do I have to ...", or "Should I ..."
"Should I need to ..." does not mean "Do I need to ...", or "Need I ..."
"Should I need to know more grammar?" means "Ought I to have the necessity to know more grammar?"
"I need to know more grammar, but I'm not sure whether I
should need to know." I'd guess that you will never have to use such a sentence.
"Should John have to know more grammar before he teaches English?"
does make sense.
"Should John need to know more grammar before he teaches English?" is harder.
I'd suggest you don't use these phrases if you think "Should I?" means "Do I?"