The term you're looking for is Pat Haba Bikisnin. This is a type of baked item which is like bread in many ways and different from bread in some other ways. According to the Shulchan Arukh (168:7), there are three different definitions of pat haba bikisnin and we hold by all three of them:
First Category: Cake
According to Ashkenazim, if there’s a majority (51%) of sweeteners such as fruit juice, oil, egg, margarine, sugar, honey, or the like in comparison to the amount of water added to the flour, then the Bracha is mezonot. However, water mixed into the sweeteners such as diluted fruited juice or margarine (usually 15% water) isn’t included in the calculations of sweeteners to water.
According to Sephardim, if the taste of the sweeteners is recognizable in the dough, then the Bracha is mezonot.
Second Category: Wafer
Dough which is filled with sweeteners such as nuts, chocolate, or the like and is cooked together with the dough, and the taste is recognizable, the bracha is Mezonot.
To be considered pat habaa bikisnin, the filling must not be a “meal” food, for example meat, fish, cheese, or vegetables (unless the pastry is clearly made to be eaten as a snack).
Third Category: Cracker
If the dough is cooked into a food that's hard and crumbles the bracha is Mezonot.
The bracha on pas haba bikisnin is mezonot when you eat it as a snack or hamotzi when you eat it as a meal. The definition of what constitutes a meal differs between Ashkenazim and Sephardim. So there are a few of incorrect assumptions in your question.
A pretzel may or may not be mezonot depending on how it's prepared. A hard pretzel fits into category 3 and would be pat haba bikisnin and would therefore take the bracha of mezonot when eaten as a snack; however, a soft pretzel is considered bread on which the bracha is hamotzi.
Whether pizza is pat haba bikisnin is debated. I don't know the specifics of the debate but I think it comes down to whether it's bread with other stuff on top or whether the whole thing is considered together and so it's not really bread. Even for those who hold that it is pas haba bikisnin (as opposed to regular bread) (which I believe is a minority), whether it's considered a meal or not isn't really determined by the number of slices you eat (as one enormous slice could easily be a meal even though it's only one slice). The two-slice rule is a rule of thumb.