For the simplicity for the argument I would stick to important games being only finals performances. This will help being consistently objective.
And I would say that any tournament that had invited at least 5 top 10 teams of the given time period should be considered a tier a tournament.
For example simples last performance in the blast series was staggering, now I'm not a big simple fan, but being an objective fan of csgo I think that was something to behold of.
So even though this was not a major, a performance like that should be taking into consideration.
Also last year's zywoo performance against navi in the final was incredible.
So you have to set the framework yourself for what a tier a tournament is, because I believe that winning a tier a tournament today is far more difficult than winning a major 4 years ago.
It is really difficult to create an objective framework, but it should be doable.
If you want I would be willing to help you once I am done with my exam period tomorrow, send me your discord name and we can put some time into this.