Nah, none of this works as an explanation
(and I'm not even sure what you're talking about when you say she destroyed her dothraki tribe in season 1).
Why? Because Varys supported Dany after everything that happened in Essos. It wasn't reluctant support either. He co-signed her. He told Tyrion that Dany was beloved by millions for her rule. In season 7, after she left Meereen, he straight up said he chose Dany because he believed in her. Nothing that happened in Essos made him think she wouldn't be good for the people of Westeros.
So again, what new information did Varys receive from the period of time where he brought Tyrion to her believing her to be a kind and just ruler, brokered an alliance between the Tyrells/Dornish to support her, and acted as her advisor during her campaign that made him go from full support of her to thinking she was a terrible tyrant of a leader, in your words.
Keep in mind, Varys is the same character that supported a Dothraki invasion of Westeros with Viserys and Khal Drogo leading the charge, back when the Dothraki culture was based entirely on raping, pilaging, and slavery. He supported this even though he knew they would have ran roughshod over Westeros. He supported this even though he knew Viserys was a piece of shyt.
Keep in mind that Dany put an end to the Dothraki raping, pilaging, and slavery. Keep in mind that she ALSO put an end to the Ironborn's raping and pilaging, by peacefully allowing them to remain as an independent kingdom to gain their support. Keep in mind that Dany spared Varys life and elevated him to one of the highest positions in her court, even after he conspired to murder her. Keep in mind that Dany just got done sacrificing half of her army to keep the realm safe from the white walkers.
With all of that on the table, when and how did he come to the conclusion that she was going to be a terrible leader that needed to be assassinated expeditiously?
I've had this conversation before, and no one seems to be able to explain Varys sudden "this bytch crazy, I gotta kill her right now and no other option is available
" transformation without
I'm interested to see if you can provide a satisfying explanation, that doesn't directly contradict series long actions and dialogue from Varys.