I have problems translating the last part of this sentence, specifically the part of んだようなものだもんね
Let’s go by parts as Jack the Ripper said. Haa! Haaa! I know that the ん means a way to emphasize a phrase, that だ is a casual form of で す
And it gets more complicated with the next word よう(this word scares me when I see it, really), using as reference the online dictionary Jisho.org we have several possible translations
よう＝ましょう We discard this for obvious reasons
and other variants that I have learned like ように、ようになります、ようにしています、ないように、etc.
This is followed もの which I suppose comes from 物
And finally we have the もんね. I understand that もん is also a variant of 物, and a second translation of もん is that it carries the sense of exclamation or surprise
After much reflection I reached this conclusion I hope it is correct or I'm wrong and it has a different meaning
寮が個室だからこの学園選んだようなものだもんねっ Well, the fact is that/ the point is that I chose this school because it has individual dormitories.
P.S. sorry if my English is not very good I did the best I could. Un saludo desde México