Hi, Vianca! As you know, ㄷ sounds close to /t/ in front of syllable like [둘], [단]점, [답]답하다. ㄷ sounds close to /d/ in the middle of syllable like 정[답], 담[담]하다 or 기[대]하다. But, actulally, the pronunciation is discriminated not by syllabic border but by intonational border. In ~면 // 안 돼요, there is one intonational border(pause) after 면. So 안 돼요 is one intonational phrase. In other words, no pause between 안 and 돼요. Thus, "ㄷ" in 안 [돼]요 sounds like the "ㄷ" in 정[답], 담[담]하다 or 기[대]하다 c.f. 답답하다 sounds /답따파다/

Trust your ears. To me, it's sounds more like the "ㄷ" in 정[답] and 답[답]하다, but not much 기[대]하다. When you speak, don't concern yourself too much with how heavy it sounds. If the 받침 before it is a ㄱ, ㅂ, ㄷ or even ㅁ ㅇ ㄴ equivalent, chances are, when you pronounce those words, this "ㄷ" that we talk about here in this question will naturally sound more heavy. It is tedious if you try to speak, or pronounce it the other way. Just my two cents.

I don't get it. As far as I know, I'm pretty sure all the consonant sounds same unless it's final consonant.