Yep, this is difficult for you, foreigners. But me like native Japanese can understand the vague meaning.
This "あれ" does not mean "That" as the word suggests.
This "あれ" suggests following sentence 夕弦の読んでる少女漫画の方がヤバめの描写多いじゃん！
ていうかあれだし means the speaker wants to suggest the whole next sentence.
But is long. So, speaker wants to say that Don't say some unthoughtful thing without too much thinking. Because, there is some reason. the reason is,
If you want to use some similar phrase, it is like this.