Mirza Ghulam Ahmad waited to do Takfir.  He didn’t start in 1891.  Interestingly enough, in 1899, MGA was ordered to never do Takfir on any opponent by the British Govt., (See Dard, pg.  662).  MGA claimed prophethood in 1901, however, he didnt issue any order on Takfir.  Further, his book Tiryaq ul Qulub was published in 1902, and it indicated that MGA and his ghost writers didnt consider any Muslims as Kafirs, simply for rejecting MGA.  However, this exact portion was written in 1899, and that was before MGA claimed prophethood.

His attitude in terms of Takfir after 1901?
As I stated earlier, he strategically, published TQ 3 years late, in there it indicated that MGA wasnt doing any kind of Takfir at all.  In fact, in 1907, MGA wrote in Haqiqatul Wahy, that he and Muhammad Ali were in court in 1904 (see page 50), for a defamation case (ref needed), and MGA and Muhammad Ali indicated that MGA beliefs hadnt changed since the publishing of TQ(1902), in terms of Takfir.  See Muhammad Ali vs. Mahmud Ahmad for further details.

1903-1904 era
MGA finally started doing open takfir.  He began to use a hadith in which it is stated that any Muslim who calls a Muslim a Kafir, he is a Kafir.  And every Muslim who was calling MGA a Kafir, was subsequently doing Takfir on themselves.

1906-1907 era
In this era, the famous Ahmadi, Dr. Khan quit Ahmadiyya.  MGA wrote a private letter to Dr. Khan wherein he did Takfir on the entire Muslim ummah, simply for rejecting MGA.

1911 era
Mahmud Ahmad quoted MGA’s letter to Dr. Khan and wrote a controversial pamphlet, entitled, “A Muslim is who he who accepts ALL the sent-ones of Allah”(see page 33).  In this essay, Mahmud Ahmad openly did Takfir on all Muslims.  Noorudin edited the essay heavily, and was openly concerned with the notions of Mahmud Ahmad.

Muhammad Ali responded with his own essay, in fact, in 1913, Noorudin exclaimed that Mahmud Ahmad still hadnt fully understood Takfir and Muhammad Ali would write on it once and for all.  Khwaja Kamaluddin also wrote an essay refuting Mahmud Ahmad.  This is where the theological differences which created the split actually began.

1915-20 era
This is the era wherein the Qadiani branch openly did Takfir on all Muslims, and they used the prophethood of MGA as a decisive factor.  Mirza Bashir Ahmad wrote extensively, in fact, the Lahori-Ahmadis argued the opposite to the Qadiani branch.

1922 era
The Qadianis stopped doing Takfir, in fact, they began calling Sunnis and Shias as non-Ahmadi-Muslims.

1935 era
The Al-fazl quoted MGA’s letter of 1906 wherein he did Takfir in his private letter to Dr. Khan which some of that can be viewed here, https://ahmadiyyafactcheckblog.wordpress.com/2017/05/07/new-research-data-books-by-dr-abdul-hakeem-khan-have-been-found-vs-mga/

“God has revealed it to me that the person who did not believe in me after having heard about me is not a Muslim.” (Al-Fazl , Qadian, Jan 15, 1935 – Al-Hukum , 4:24, Mirza Ghulam Qadiani

1954 era
Mahmud Ahmad is ordered to come to court and answer questions about Ahmadiyya.  He denies all Takfir.

1965 era
Ahmadiyya leadership has a book published vs. the Lahori-Ahmadis, they deny that Mahmud Ahmad ever did total-Takfir….they call it smaller-takfir….SMH

1974 era
Mirza Nasir Ahmad is ordered to come to court and answer questions.  He denies any and all Takfir.

2004 era
The letter of MGA was recorded and consistently put into every edition of Tadkhirah, 4 editions were published.  Essense of Islam is collected and published by the Qadianis, the same reference is authenticated.

2009 era
The Qadianis removed the letter of MGA from Tadkhirah. Qasim Rashid and Kashif Chaudrhy start calling it a fake, since they cant find the reference in the new edition of Tadhkirah.  However, I was able to get a screen shot of the 2004 edition of Tadhkirah and and have compared it with the 2009 edition, and these Ahmadis are caught red-handed.