بسم الله الرحمن الرحيم
It is enough to be an Imam to remember some surahs and read them with correct articulation. It is not mandatory to learn the whole Quran by heart. Thus the person mentioned in the question who is Alim and Qari it is valid to offer salah behind him without karahat and salah shall be valid. If you get someone who can lead tarawih salah in Ramadan without any charge then let him lead the tarawih prayer; otherwise this imam should lead tarawih salah from Alam Tara (Surah Inshirah) as there is no problem in it.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.