Brother Abu Hassan has already said ''...on the Divine Name al-Karim, qurtubi says in his al-asna something that is similar to the urdu lines quoted.''
So the door of the Christians is well and truly here
it seems you are concerned about "gulow" or exaggeration. don't be apprehensive.
indeed, you cannot truly praise the Holy Prophet, as is his right. the best of scholars have already said that you can praise the Prophet as much as you can, except calling him "Allah" or "the son of Allah" (like the Christians did for Syeduna Eisa).
Ala Hazrat, the Reviver of Islam, says:-
Ay Raza khud Saahibe Quran hai muddaahe Huzoor,
Tujhse kab mumkin hai phir midhat Rasoolullah ki?
O Raza, the Owner of the Qur’an is Himself the Glorifier of the Prophet!
So how is it possible for you to rightfully praise Allah’s Apostle?
Ala Hazrat uses the word Kareem to explain the unfettered generosity of the Holy Prophet.
Mere kareem se gar qatrah kisi ne maanga,
darya baha diye haiN, durr be-baha diye haiN.
When someone has asked a drop from my generous prophet,
He has given flowing rivers, he has given priceless pearls!
Perhaps the following couplet may dispel your concerns, for he explains that Allah ta'ala is more generous than the Holy Prophet:-
Hum bhikaari woh Kareem, un ka Khudaa Un se fazoon
Aur naa kehna nahee aadat Rasoolullah ki!
We beg from him, the generous, and his God more Magnanimous than him,
And saying “No” to the beggar, is not the habit of Allah’s Apostle!