Is it permissible to pray 20 rakaàth of tharaweeh after ishah salah and 5 or 7 rakàth of witr in the last part of night?
From a fiqh perspective what you quoted is something permissible. But it has no exact backup in the sunnah of the Prophet (pbuh).
Witr is an odd prayer one prays as the last prayer in the night, it might follow a set of two raka'ah prayers (at least this is the view of the majority of scholars) which is either called shaf'a during all months of the year or one may call it tahajjud in Ramadan especially (however some call the last two rak'as shaf'a even in Ramadan).
So usually in Ramadan you pray taraweeh 8, 20 etc raka's in the sum right after 'isha'. And after having a nap you may pray tahajjud as a set of two raka's and end it with witr.
I'd like to know about Tahajjud Prayer in detail
Is sleeping a necessary condition for Tahajjud (night prayer)?
Is it okay to perform Tahajjud after Witir?
Does Taraweeh replace Tahajjud prayer in the Month of Ramadan?
Can one pray Tahajjud later in the night after they have prayed 20 rakah Taraweeh
Can I pray tahajjud with only sura Fariha, and sura Ikhlas?
Step by step on how to perform tahajjud prayer
Witr salat with isha and tahajud
Reference of a hadith about the night prayer of The Prophet pbuh
❶ - Yes, you can pray Taraweeh/Tahajjud in any amount, based on the following hadith which the scholars used to indicate the permissibility of praying more if you wish:
“The night prayer is two by two. So if anyone fears the break of dawn, he prays one raka’ah making uneven, the number of what he had prayed” (Bukhari).
❷ - Witr means odd, so you can pray any odd numbered prayer, as per the above hadith.
You might be interested in the following list of authenticated voluntary prayers: https://abdulmaliklive.wordpress.com/2016/07/11/sunnah-salah-voluntary-prayer/